Almost nothing counts as a basic attack, why would you have a piece of armor that only affects a tiny amount of champs that rely on basic attacks. I would like a change in this armor to reflect all AD attacks at 35% and AP at 10% (it would be the only stat that would help mitigate a fed kayle)
Like Tyrion said, it's a niche item. That is, it isn't meant to be a standard buy for any occasion. You build it to deal with specific champions that are completely auto attack reliant but have insane snowballing (ie. Tryndamere, Fiora, Xin Zhao, Any Marksmen not being played as an AD Caster, Master Yi, Aatrox, etc.
Furthermore, it's not meant to be built buy everyone. It's an item for champions that are already tanky. It's considered core on Rammus because he has a taunt to force champions into auto'ing and is generally the go-to champ for Snowballing aa champs.
If they expanded this item to reflect all AD attacks, tanks or pseduo-tanks that built this would become insanely powerful and Marksmen would drop significantly, leading to a pretty big off-balance.
If you have AD Champs that don't rely on auto's, build a Randuins or Sunfire Cape instead. Almost nothing counts as a basic attack OTHER THAN ACTUAL BASIC ATTACKS THEMSELVES.
It's a niche item. It's in a good spot and is a good buy vs certain champions/team comps.
Thormail is usually my last item when I play a tank againts an all AD team.
I love this item. I counters most adcs, whose main attack is their basic.
