It would be very challenging to have a game mode in crucible where you can pick up the weapon of the opposite player whom you just killed. When the opposite player returns he/she will not possess that specific weapon to continue, so the player will be forced to use another weapon (not the favorite one). on the other hand the one who picks up the weapon will have a chance to use (for a short time) a weapon that is not in his/hers arsenal. I think this will change the strategy of most players. The idea may be developed to randomly grant an exotic or high profile weapon someone who picked up the most number of weapons during one life etc...