Crit damage seems great. At first I thought it would be awesome. Then I checked the math. I have not looked back since.
First of all, Crit Damage fully relies on having a chance to crit to do *anything*. Any other rune would be benefitting you all the time! It also means your item build is locked-in, and your effectiveness is purely late-game. You *have to* go crit chance in order to make it effective, meaning you can't adapt to the game as it progresses.
Second, only three champions benefit inherently from Crit Damage. Shaco can benefit because he starts at level 1 with a skill that makes him reliably Crit. Ashe's passive gives her somewhat reliable Crits as well, but is still quite conditional - realistically the passive only affects the first attack of any fight. Tryndamere ultimately buys himself into high critical chance late game, but he already has a critical damage ability (gives him 36% additional Crit Damage,) making further Crit Damage less powerful.
Third, and probably most convincing, Crit Damage is virtually always worse than Armor Penetration. It is obviously vastly more effective on normal hits, but it will give more damage EVEN ON CRITS! (See my rune database spreadsheet for reference for rune statistics: https://www.leagueoflegends.com/boar...ad.php?t=50741)
In Marks, you're trading 17.64 Armor Penetration for 20.07% Crit Damage. In Quints, you're trading 11.73 Armor Penetration for 13.38% Crit Damage. The sum of those is: 29.37 Armor Penetration for 33.45% Crit Damage.
This is how damage is calculated on a crit:
Final Damage = Damage * Crit Damage * (100/(100 + armor - armor pen))
So let's assume the opponent has around 30 armor, perfectly normal early game. Since the amount of damage you're doing is irrelevant to the comparison, let's say it's 100 damage for the sake of simplicity:
With 33.45% Crit Damage: 100 * 2.3345 * (100/130) = 179.58 damage
With 29.37 Armor Pen: 100 * 2 * (100/100.63) = 198.75 damage
That's right, Armor Penetration did more damage on a Crit.
But, you say, people have more than 30 armor. Alright, what if they have 100 armor?
With 33.45% Crit Damage: 100 * 2.3345 * (100/200) = 116.73 damage
With 29.37 Armor Pen: 100 * 2 * (100/170.63) = 117.21 damage
That's right, Armor Penetration did more damage on a Crit. Again.
But, you say, late game people can have a ton of armor. Alright, what if they have 200 armor, and you have an infinity edge?
With 33.45% Crit Damage: 100 * 2.8345 * (100/300) = 94.48 damage
With 29.37 Armor Pen: 100 * 2.5 * (100/270.63) = 92.38 damage
That's right, Armor Penetration almost did more damage on a Crit. Late game.
a) What about Last Whisper?
Percentage armor penetration (such as Last Whisper) was recently changed to apply *after* flat armor penetration. This makes flat Armor Penetration somewhat less effective with it. However, you will not always have a Last Whisper. Even if you do, flat armor penetration still retains 60% of its prior effectiveness. You shouldn't primarily be going for high-armor targets, anyways. Who ever pings the tank and says, "KILL THEM FIRST!"? And, as is obvious, you won't always be critting.
b) What about Black Cleaver/Stark's Fervor?
If your team has reduced the enemy to less than 15 armor, then yes, Critical Damage will do (slightly) more damage ON A CRIT. But, I doubt that when you engage they will always be below 15 armor. Black Cleaver requires you to hit them a few times, first. In addition, you would need to have an ally with those items instead, othewise you wouldn't have any chance to crit. Unless it's very late game, in which case it's doubtful your enemies wouldn't have bought ANY armor items against a fed physical DPS. On the other hand, if they are merely reduced to low armor, your flat Armor Penetration runes just got even better!
c) Your example had someone with just 100 damage. Late game, I have way more than 100 damage! I'll be critting for tons more!
Indeed, you will be. So will the person with Armor Penetration. They'll still be doing more damage than you.
d) I always get 100% chance to crit, so Crit Damage is better!
Nope. The calculations you see above are ON A CRITICAL HIT.
f) Armor Penetration increases DPS. Crit Damage increases your burst damage. I'd rather have giant crits than simply increase my DPS.
Nope. The calculations you see above are ON A CRITICAL HIT. Therefore, Armor Penetration increases your "burst damage" more than Crit Damage!
e) I play Shaco or Ashe and when I go in for First Blood, they don't have that much armor, so Armor Penetration is a waste!
Then use Flat Damage Quintessences. They'll increase your first attack out of the Decieve by more than Crit Damage. Indeed, it will increase ALL of your attacks' damage.
f) Why would Riot make Critical Damage runes worthless? Why did they nerf them after they were already extremely bad?
I don't know, ask them. They don't seem to update rune stats often at all. One problem is that once you buy them, you're stuck with them. If they nerfed a certain set of runes, people might suddenly find themselves to have wasted all their IP on useless runes.
g) What about Seals and Glyphs? There's no Armor Penetration for them.
There are much, much better choices than Crit Damage for Seals and Glyphs. Try HP/lvl, Dodge, or Mana Regen/lvl for Seals and Flat CD red, flat/per-level Magic Resist, or flat/per-level AP for Glyphs. You'll be handicapping yourself if you don't go with primary runes. Not sure what the stats are for various runes? Check out my Rune Database Spreadsheet: https://www.leagueoflegends.com/boar...ad.php?t=50741
h) FUUUUUUUUUUUUUUUUUUUU... I wasted so much IP on Crit Damage runes!
I'm sorry. :/ Sometimes the truth hurts. It's okay, it's just a game. Give yourself a bit of time and you can earn some new runes. If you want, you can combine them away, or you can keep them in case Riot buffs them. Your choice.
i) Who cares?!?
I'd prefer to have a post to point to when I see someone ignorantly proclaiming a Crit Damage page's awesome power. Plus, it's fun for me to analyze the game and articulate my argument.
j) You can only get Critical Damage from one very expensive item, whereas you can easily get Armor Penetration. Therefore, Critical Damage is better!
No, quality is not determined by rarity, but by valuable attributes. Crit Damage may be rare, but it's still terrible, and worse than Armor Penetration. Pet Rocks are rare, but they aren't worth anything to anyone intelligent.
tl;dr: Don't ever buy Crit DMG runes. If you want to get Crit DMG, get Armor Penetration instead.