So as you can (hopefully) see in the game above, the move in question is Bg5 and its counterparts. What I don't get is why high(er) rated players still play this move. I have always thought that Bg5 was a weak move since a) It trades a bishop for a knight, b) It allows Black to get another g-pawn, strengthening position and c) In the best case, it allows Black to fianchetto the bishop after taking with the pawn. Is there some sorta hidden disadvantage or tactic here, or am I just tripping?