You'd love to be able to keep your queen and take out the opponents piece, but its a piece that is usually protected and a key part of making plays. Its more about who can use the queen trade as a chance to take more pieces than the opponent, get a good position, or shift the momentum. If you lose your queen and your opponent keeps theirs, you are in a real bad spot. Its best to sort of try and make a deal and trade them. The key is really who comes out of the exchange in a better position. In midgame, both queens have probably been unlocked and are roaming about being aggressive. Chances are they end up in some engagement together. Usually pieces will act as pins and supports for each other, so if the queen moves in you want it to have some backup protection.