Taunt exists as a mechanic on 3 champions. You really want to make an item that only 3 champions will ever build, period?

Now if it had an active that allowed you to taunt...

Well technically it's 4 champions (ahri, shen, rammus, galio) but your point still is valid

The way I see it, Thornmail is a niche armor item designed to facilitate the dueling of fed AD carries lategame.

Except you don't auto attack while "Charmed", which is why it's called that and not "Taunt".

You forgot Teemo. Dat Global Taunt.

@RiotNome, I'd like to direct you to my post here, http://na.leagueoflegends.com/board/....php?t=2786000 which details the breakdown of Thornmail to Frozen Heart and shows that even for champions whom recieve no benefit from mana FH is still by far the superior choice in any game mode due to the CDR and slowed enemy attack speed in all but strict 1v1 stand-and-fight situations, not escaping or otherwise maneuvering on the battlefield.

What I don't understand is why the Aura reduces armour more than magic resist, surely it should be even or the other way around? Bruisers have loads of MR options and Armour pen options, even more in season 3. Why are casters getting the short end of the stick?

Chronicle, it's percentage.

Tanks and tanky dps champions tend to have more armour than MR, in almost every case.
So to avoid tanky derps fighting each other for hours, they have more ArPen%.

Mages are less affected by the ArPen%, since they have lower ARM to begin with.

That may of been the intention but it is not the case. All it does it make mages weaker when they try to itemise vs bruisers to not die and gives bruisers/adc an even stronger lategame that they do not require. This will be an even bigger problem in S3 where the aura is applied first, since it will make mages even weaker to flat pen than they already are. There really is no need for it. If they want to remove the tanky derpness they shouldn't have an Arpen or Mpen aura, they should just give armour/mpen stacking diminishing returns same as movespeed. Every point of armour/mr over X amount is worth 80%, then every over Y amount is worth 50%.

The needs of the many outweigh the needs of the few.

I suggest we hold this discussion 1-2 months after S3 comes around, due to the increased cost/reduced efficiency of resistance items across the board. We'll take a couple of real cases, see how many resistances people have, and how the penetration aura affects them.

People in my experience tend to build a lot more armour than they do MR.

Also, the bruisers are the many, hue hue hue.