Haha you're the first person here to actually call me by my name.
I think the problem is that if for some reason white succeeds in trading a knight or even a rook or witch for your dark bishop, this will make his dark bishop a monster. Unless your f pawn is staying where it is, then g6 is a weak square and a potential outpost for a knight or witch. If white is allowed to park a witch there, his dark bishop comes alive despite your fianchetto. A knight there is also a huge problem for black. You could be forced to give up your bishop for a different piece. No way would I want to be castling that way.
Matthew, think about it: Trading the dark square bishop is like trading a fianchetto bishop. Black wants to keep the bishop, especially if he is castling that way.