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?

Thoughts?

Counter building is actually the best way to play, people just dont have the mind set to do this though cuz they follow cookie cutter builds that they had tested and done well or someone says its done well.

but yes, if people learned to counter build properly then they would be a better player, im sure all top league players all counter build and knows this.

though I wouldnt go to the extreme of everyone on your team building a specific item or what not because not everyone will be targeted in the team fight

to counter build effectivly you gotta use the death cap to see what killed you and how to protect yourself from it.

I analize what champions are in loading screen. My first buys are, most of the times, always the same. If enemy mid is going strong i build MR; if enemy top or bot are going well, armor; but keeping in mind my role.

Imagine your mid is doing bad, with Garen i build Hexdrinker / Maw of Malmortius (MR, magic barrier and attack / Maw: more attack while less hp). You keep your damage output and MR up while you still are in your role.
Imagine your mid is doing well, with Taric i build Abyssal Scepter (AP and MR, plus reduce MR on enemy champs). With these, you will have a little more MR, AND you give your mid the chance to harass more when lane phase is over. Plus, you will have more AP for your skills. And you keep in your role.

Remember, analize the situation of your team and enemy team.