It was a discussion of which was better between Gunblade and WotA, and the answer to that comes down to "how often do you get to use Mace of Spades?"

It's not derailing, it's focus on a single aspect. Derailing would be changing the discussion to how Rumble is basically an overall stronger AP caster, lane bully, and far less reliant on his ultimate (even though his is also insanely powerful). THAT would be thread derailment. Another way to derail the thread would be to point out that we're technically discussing a champion rather than items.

I suppose this would depend on . . . average enemy health. Lichbane (in your build) is worth about 500 bonus damage (before reductions) every 2 seconds ('cause Mord isn't a "1-skill toon"), while Liandry's is worth (at best) ~50% of their health, assuming nothing overrides Mace of Spades (and/or that CotG procs as a non-DoT), and that you spam Q, and they're slowed/stunned/snared for the full duration (20 seconds). Getting that 40% requires that, while the target is "mobility-impaired," you hit them with each spell in succession (getting Q on them as a solo proc), spacing them all 3 seconds apart, and getting a 2nd & 3rd Q in during CotD's duration.

So over this hypothetical 20 second interval, you could either do ~500 x 10 = ~5,000 damage, or ~50% of the enemy's HP; 5k hp / 0.5 = 10k, so when the average enemy hp is (effectively) 10k+, Liandry's will be better. Note that this applies to 5000 hp tanks w/ 100 MR as well as 2000 hp mages w/ 400 MR, and everything in between.

Because of how Mord tends to work, and the woeful lack of lasting CC, Liandry's will be less efficient on Mord; he is more likely to get 15-20% out of Liandry's (at best), and the lower the %, the more hp/mr they have to have (on average) to make it more effective. Liandry's works best (and was designed for) champions who prefer to "lane," such as Vladimir.

EDIT: I just realized that Liandry's is base 5% current (rather than max), which means that all of those best-case percentages have to be halved again. Bottom line: Lichbane is still better than Liandry's.

Morde only pokes in lane, he needs burst in teamfights because his one and only job is to get a ghost. You want items like Abyssal, DFG, Lich Bane that give you maximum burst onto their squishies so you can get a ghost.

i like building rageblade on morde, liked it before it was changed and even more so now. he gets to 5 stacks pretty much instantly. i would guess it does more dps, especially with a gunblade, than lichbane and you don't have to buy a mana crystal either.

liandrys for health teams, lichbane for squishy teams.
Simply, Lichbane will be better for bursting squishies, and liandry's is when they have a very healthy team.