say 300 armor to start (200 bonus plus 100 base):
first thing that gets applied is the other tank's black cleaver: You're down to 210 at 6 stacks.
Then you apply LW which takes off 49.5..we'll round to 50 for ez maths: You're down to 160 armor.
Then factor in 85 lethality at level 18: You're down to 75 armor.
So from 300 armor you get an effective 75 armor.....or a reduction of 225 armor. That's just shy of two thorn mails and a ninja tabi's worth of armor.
Or the armor from 4.5 deadman's plates.
Or using the gold value of armor at 20 gold per 1 armor, thats 4500 gold worth of armor washed away by pen items. Thats already ridiculous.
There are two things people forget about these stats as well: First, the % armor and bonus armor stats become increasingly more efficient as you stack armor.
**Second, Each point of armor removed increases effective damage by MORE than the last point of armor you've removed. This make armor pen stats and magic pen stats the only stats in the game that actually have INCREASING marginal returns when the stats they are made to counter in fact have diminishing marginal returns.
**
This is second point probably the most ridiculous aspect of how armor pen stats work. You can't have a stat that has increasing marginal returns counter one that has diminishing marginal returns without there being a huge mismatch in the power of those stats at different extremes. The extreme of high armor vs no armor pen isn't necessarily an issue because early game before people buy their pen items because tanks wont have 300 armor or anything close to it that early. But when you get to the point where you can attack a 300 armor tank as if he has 75 armor late game, you have an issue. Oh any by the way, generally speaking anyone with 300 armor has likely bought nothing but armor and therefore either has low health (for a tank) or low MR. In a world where armor pen doesn't exist, 300 armor on a 3000hp tank gives that tank 12000 effective hp vs physical damage. With the effective armor at 75 this is reduced to ~5200 effective health vs physical damage. Again, remember this is on a tank who is absorbing damage from multiple people at once. It is unbelievably easy to mow down that kind of effective health, even assuming an all AD team. Throw in a good damage mix, its even easier because that 300 armor, 3k hp tank likely has shit MR. So one full rotation from a mage will likely knock off about 1500 effective hp by itself.
Simply put, There is a point where it no longer even becomes useful to buy armor. With all the lethality that was put in the game, we've basically hit that point. Before all these new items, the max flat armor pen was what...35ish? we'll use 35. That means in that same scenario, that tank has 125 armor after all armor pen stats are applied. That would give that same 3k hp tank an effective health of 6750. Thats about 1500 more effective health which is quite a bit more time he can stay alive if you consider everything is going to be on cooldown for the other team after the first 5kish effective health (again, this is vs only AD and incorporating AP into this just makes it worse for the tank).
Ok, your argument then becomes, "But they can just stack health and those armor pen items wont really matter." Thats not true either. Here is a list of champions that prove that (may have missed a few):
Amumu
Brand
Dr. Mundo
Elise
Evelynn
Fizz
Garen
Gragas
Jarvan IV
Jinx
Kog'Maw
Lee Sin
Malzahar
Maokai*
Mordekaiser
Nasus
Nidalee
Poppy
Sejuani
Shyvana
Trundle
Varus
Vayne
Vi
Warwick
Xin Zhao
Zac
Zed
And here are items that also prove it:
.
What do all these things have in common? Damage that scales with enemy health. So players can build items and play champions that do %health damage, effectively countering stacking health. And those same champions can also build a bunch of pen item that effectively counters armor.
End result is that tanks are left with zero counterplay to incoming damage other than saying screw tanking and playing an assassin/mage/diver.
This is why i highly suspect the tank rework will end up giving tanks lots of % damage reduction, shields, and/or healing. Because as things are right now, itemization and champion abilities have destroyed the idea of what it means to be a tank.