I don't claim to be amazing at league. In fact, I'm certain that I play people much more skilled and experienced than I on a daily basis.
I have a question for the good people here on G&S.
When my friends and I play, and we see an abundance of magic damage or attack damage champions on the enemy team, or if the only fed champs are one or the other, we stop our builds and grab a thornmail / Bulwark(? the new highest MR item in the game)
In general, it works out pretty well for us.
Now, we play league to have fun. Sometimes, five of us will group up, and que in for normals, and we'll have a themed game. All poison characters, all tanks, all ap carrys, all undead, etc.
We don't expect to win when we all play the same roll of champion. We know that if we saw Vayne/Caitlyn/Graves/Corki/Tristana, we would all buy a thornmail each (right after ninja tabi). If we saw Xerath/Swain/Fizz/Brand/Morgana as our enemy team, we would again, ignore our build and counter the enemy team.
Why do most people not do this?
We do well enough.
Is it inefficient and we're just getting lucky?
Do you counter your opponents' builds?