Several reasons. I want to pick up an object and not have that act affect my player. If I set a contraint, then the object is locked into my player physically and I get dynamics I don't want.

By disengaging/unregitering an object, my hope was that I wouldn't be losing processing power to the physics engine since my held object would be just blindly attached to the player by a ent_move command. I feel this actually uses less resources than relying on the PE.

As I've posted, this disengagement approach works perfectly with the exception of residual velocity (or maybe momentum??) at re-engagement.