Exchanging pieces aimlessly is a bad thing indeed, especially if it results in speeding your opponent’s development, relieves the pressure on his position or cedes an important positional consideration. Before you exchange, you should always ask: Will I be better or worse following the exchange of pieces? For example, I will readily trade both my Knights for the bishop pair if I think it will improve my chances of winning, even if there is no immediate gain of material (he who has the bishop pair has the future). However, I'm just as likely to cede the Bishop pair if it results in a won game for me. The point: exchanging pieces must have a purpose.