Xerath is a poor siege mage compared to other mages with long range because:
1. He has low dps with which to poke down enemy team's hp.
2. He can't zone out the tower for the ranged carry to poke the tower.
3. He can't make a lucky catch on an enemy squishy or followup well on an ally's lucky catch.
4. He's just on par if the enemy team decides to rush his team, at which point the siege is broken anyway.
This post examines Xerath in his intended playstyle, which, according to Riot, is a siege mage with constant damage:
Hey guys, here to explain what happened with Xerath and provide some updates
First, the ultimate change was intended for a number of reasons:
1) It brings Xerath’s playstyle closer to what we originally envisioned with the character (Constant siege barrage instead of instant burst)
2) The animation was broken with the previous ultimate and we wanted to fix that
We were also aware that this was a significant nerf to Xerath and intended to buff him at the same time to compensate for it, what we did NOT intend was the animation changes going in without the buffs accompanying it.
We will be giving Xerath the intended buffs in the next patch. We messed up on this one and I apologize on behalf of the team to all the Xerath players out there.
How does Xerath do as a siege mage? My assessment is that Xerath is an awful siege mage compared to other long range mages because he is outclassed in traits that are desirable in siege characters. Let's compare him in terms of four factors that are important to a siege mage: 1. dps--how much damage output can he do in a poke; 2. zone denial--how well can he clear an area for a ranged dps to poke towers; 3. high-risk reward--how well can he catch an enemy defender out of position or follow up on a team mate catching the defender; 4. anti-rush--how well can he punish the enemy team for trying to jump his team. As you'll see, he's very weak in the first 3 areas and just on par in the last.
A. Defining the Characteristic
The most basic characteristic of a siege character is his dps on his harassment skills. After all, you want to poke at the enemy defenders as much as possible. Certainly you don't want Alistar to mash his E and just heal all the damage you did. Let's compare the dps of their poke skills.
I have here a spreadsheet that extensively covers the dps and burst of these characters:
Spreadsheet of dps: skill dps fixed.xls
Note that not all the numbers here are relevant to this topic. For the purpose of this topic, the page you want to examine is named "no shield," and the dps of the poke skills. Thus, you should ignore the total, which includes all skills, even those that aren't poke.
In the sheet, I assume that the mages have 600 AP, but Xerath has 660, with the 60 AP lead from the fact that his W allows him to skip Void Staff (1.3*(80/1600*2295-70)~60, using NLR as the base cost.
I've also assumed 34% cdr: 4% mastery, 20% blue buff, and 10% blue elixir.
The spreadsheet is linked such that you can simply change a single number to change all the AP for all sheets and characters. If you feel my assumed AP and/or cdr isn't reasonable, it is easy enough to play with the spreadsheet and put in your assumptions.
B. Poke Skills
Q is his only poke skill because it is the only low cd long range skill.
Q and W are both poke skills. E is difficult to use with her Q range, so it's not included.
Q is his only poke skill because the other skills have too much cd to be expended on poke.
E and R are her poke skills, because R has a low cd. Q can technically be part of the poke, but it's unlikely to hit anyone but tanks, and if Q does hit, then it'll probably be an initiate.
C. Poke Power
235 base, 0.6 ratio, 5s bcd
71 base dps, 0.18 ratio dps, 191 total dps
Xerath Total: 191 dps
144 base, 0.32 ratio, 4s bcd (taking the average of 80%, when it lands on a champion)
73 base dps, 0.16 ratio dps, 170 total dps
250 base, 0.7 ratio, 9s bcd
42 base dps, 0.12 ratio dps, 113 total dps
Orianna Total: 283 dps
255 base, 0.65 ratio, 4s bcd
97 base dps, 0.25 ratio dps, 244 total dps
Ziggs Total: 244 dps
240 base, 0.6 ratio, 10s bcd
40 base dps, 0.1 ratio dps, 101 total dps
690 base, 0.75 ratio, 40s bcd
26 base dps, 0.03 ratio dps, 43 total dps
Lux Total: 144 dps
Ranking of Poke DPS
As you can see, Xerath beats Lux's dps by roughly 35%. However, Orianna beats him by about 50%, while Ziggs beats him by 30%. For a siege mage, his pure dps is actually quite unimpressive, only managing to beat out a notoriously low dps mage whose high cds hold her to subpar levels.
For those who are wondering, if Lux's Q is included into her poke, then her dps rises to 235 on the primary target and 190 on the secondary, beating out Xerath on the primary and tying on the secondary. However, I don't think it's legitimate to include Lux's Q into her poke dps.
Let's look at whether Xerath has other characteristics that compensate for his low dps, at least in the siege role.
II. Zone Denial
A. Defining the Characteristic
There are multiple ways to successfully siege. The most basic is to poke the enemy team down until they have to back off. However, a second way to siege is to gradually poke the tower until it's destroyed. These are by no means mutually exclusive. For tower poke, the most important characteristic is zone denial: Getting the enemy team to back off the tower so that your ranged ad can take a few potshot at the tower when the creep wave comes in.
B. Zone Denial Skills
Fear of getting damaged is, of course, one part of zone denial. However, because zone denial has to come with the creep wave, the damage is more likely to come as a burst. Furthermore, because creep waves arrive every 30s, more skills can be used for zone denial than for poke.
Q damage only. His E has too little range to target a squishy. His R doesn't come out fast enough to nuke down a character, nor does it have a low enough cd to be used for zoning.
Q and W damage. Can leave the ball near the tower to set up for R to zone out squishies from clearing creep waves.
Q damage. E can be left near the tower to prevent squishies from walking forward. W might be used for the same purpose to cover the entire lane, though it might not be recommended because it leaves his team open to being jumped. Neither E nor W is really used to damage people.
E and R damage. Alternatively, leave E near the tower to zone out squishies, because it sets up for an easier Q, in which case R would lose 190 damage from its base, if it should be used at all. Q should be saved to catch a squishie out of position if possible.
C. Zone Skill Damage
Orianna: 922 (taking 60% damage for Q)
Lux: 1740 (E and R both for damage) or 950 (using E as a slow field)
Because Xerath lacks any skill that can stay on the ground and punish opponents for getting into the area, he relies entirely on his burst damage to zone out enemies. However, his Q is very low--much lower than what Orianna can deal with Q+W or Lux with E+R. His R can't be spent on this, because it has too long of a cd. Compared to Orianna leaving the ball for R initiate, Ziggs leaving a slow field for easy initiate, or Lux doing the same, Xerath simply has no zoning potential. He relies entirely on a good initiator on his team to be of any threat whatsoever around the tower.
III. High Risk Reward
A. Defining the Characteristic
Basically, this is what happens when you get a lucky skill shot into someone--say the ranged carry stepped forward to lifesteal off a creep, or their AP moved forward to clear a wave, and you hit with something just right.
B. High Risk Reward Skills
Essentially nothing. To do anything other than damage, he has to first lane his E, which at 900 range rooted is too risky for him.
AoE pull (~1s hard cc) and AoE slow field.
AoE slow. Theoretically, he can get the knockback on his W, but it's highly unlikely for it to knock an enemy forward into his team.
Single target root (2s hard cc).
Xerath has essentially no way to punish an out of position enemy by himself. Even if a team mate catches an enemy out of position, Xerath's R now comes out too slowly to follow up with burst. Compare this to Orianna's strong initiate, Lux's ok initiate, or Zigg's slow, Xerath has essentially nothing.
A. Defining the Characteristic
Sometimes the enemy has strong initiation and want to jump you while you are sieging them. This is the ability to punish them for moving in. There are two separate ways to deter rushing. One is burst damage--how quickly the mage can burst down enemy squishies if they get too close. The second is counter-initiate--how well the mage can distrupt an initiation.
Again, you can use my spreadsheet to get a good idea of the kind of damage Xerath and other mages can put out with your own assumed numbers.
Spreadsheet of burst: (actually the same as the previous one) skill dps fixed.xls
B. Anti-Rush Skills
Passive armor makes him durable against tanks.
R AoE counter-initiate. W slow field.
W pushback. E slow field.
E slow field. Q root.
C. Burst Damage
Shield Single: 480
AoE: 2425+160 per extra mine
AoE Shield: 780
Rankings of Burst
Ziggs: 2425+160 per extra mine
Everyone else: 0
Orianna and Ziggs: 0
Everyone else: 0
Believe it or not, this is where Xerath's passive really shines. Because he's a long range mage with high burst, it's hard for the enemy carries to hit him. That leaves bruisers to hit him. However, bruisers can't buy LW in general, meaning they will have significant problems trying to kill Xerath through his extra 100 armor.
Xerath has higher burst than everyone. However, his AoE is only slightly higher than Ziggs', and it's really the AoE that counts, because Xerath most likely isn't in range to use his E on squishies. Nevertheless, it's much more impressive than Orianna's or Lux's.
Overall, though, Xerath's personal durability and high burst really only puts him on par with Orianna and Ziggs, because he lacks any disruption or protective abilities.
V. Final Thoughts
Xerath isn't a good siege mage. He lacks the raw dps to poke down the enemy, because he can only use his Q, which has poor dps. He has no way to zone out the tower when a creep wave comes in so that his ranged ad can poke the tower. He can't make a lucky catch on an enemy squishy, and he has problems following up an ally's lucky catch. His only saving grace is if the enemy decides to dive his team, where he can offer a lot of burst and is hard to kill. Even then, he's not necessarily any better because he doesn't offer any disruption or protection. Xerath simply lacks any strong points that would make him a good siege mage.
VI. Useful Thoughts
These are detailed posts on the subject in this thread that you may want to read in depth. I don't necessarily agree with them, but they provide useful discussion. It's still useful to read other posts in the thread, because even a single sentence comment might give you useful insight into the subject.
 WarlordAlpha on Xerath's high risk reward v. anti-rush abilities.
VII. Edit History
PDT; 2012-07-11; 16:20: Fixed a spreadsheet error that used 0.7 AP ratio for Xerath's E. This changes his single target burst for the anti-rush section, increased by 65. The fixed spreadsheet is attached as skill dps fixed.xls.