It doesn't account for the magic damage in a lot of DPS builds, and even at that point, it still has a counter in 
. So basicaly, I built thornmail to counter your DPS which is mostly magic damage! Well ok, I'll just...umm...continue to beat your ass because I don't care?
Or, I built Thronmail to counter your Physical damage! Well ok, I have way too much lifesteal for that to even matter, and also, if that's not enough,
EndlessSorcerer4 votesYou build Thornmail primarily to prevent the enemy autoattackers from sustaining through your damage when fighting them. It's not meant to make you invincible or to kill them solely through the return damage.
If you are a tank or fighter with another major armor item and a good amount of health, it can be it quite difficult for autoattacker to actually fight you.
If you want some examples:
You don't rush the item, but it's quite useful for reducing the enemy champions sustain through lifesteal. With that factor removed, you should be able to duel them rather reliably.
AyRe CoNteMpT3 votesi'd say thornmail is one of the top 3 defensive items in the game but i accept that it takes quite some skill to understand its strength, so i wont be mad at you for saying that.
Tract1 votesNext time you fight a pure tank such as Rammus who has thornmail on, you will quickly realize that its not useless.
Jbels3 votesNext time you fight a pure tank such as Rammus who has thornmail on, you will quickly realize that its not useless.
Rammus has 2 Thornmails, and the one he has built in scales better than the actual item. Being good for one champion doesn't make the item good
Tract1 votesThat was one example, I can list many more if you want? Maokai, Voli, Sej, Ammumu, Nautilus, Renekton, the list goes on. Thornmail is far from useless.
Big Lincoln1 votesAre you saying it doesn't account for magic damage because it's an armor item?
Because Thornmail considers -all- damage in an auto attack when it reflects it.
And no, Thornmail isn't useless. You just can't stand there doing nothing and expect it to work for you.
mye PP1 votes(
or 
)+ + Magic Pen
Just fixed your problem