By "wrong", I mean that building defense to try and counter said Champion ends up just shooting yourself in the foot most of the time. Examples of this would be Vayne and Fiora (%-max HP true damage), but you could also include Yasuo due to his 50% bonus ArPen ult steroid. Armor against those Champions simply does squat in most scenarios, and building HP usually has the same result as well.
So who else could be considered on this list?
CrazedPorcupine1 votesVeigar
MistroTTC1 votesVayne and fiora still do mostly physical damage, you still need to build resistances against them, also yasuo only has the steroid when he ults, doesn't have any percent hp damage without Botrk, and 50 percent bonus armor pen isn't enough to make armor useless.
To answer your question it would be mr against an all physical damage team against trundle, anything else is at worst a waste of gold.
darkdill1 votesSo is my question a "wrong question"?
Takazaki1 votesLoooooooooooooooooooooooooooooooooooooooool @ yasuo
no armor makes his crits more effective also his passive does not pen item effects like 
ModAttysu The Poro1 votesI don't think it's completely useless, more of just highly less effective...
Violet Fields1 votes