Reincarnation
The ending of this fine movie is sad in a way,but there could be no other end. It's like reincarnation, it would be impossible to live each life anew if we remembered who we were in other lives. It's hard enough being one person!
If Tom remembered Joe's experiences, it would be as if he were possessed, or schizophrenic. If Joe consciously occupied Tom's body, he would think he was insane, and would be totally nonfunctional.
Only one consciousness per body is the best rule; besides, Joe still had his Love and his best friend who knew the truth, once they compared notes. They would guide the spirit they knew to be Joe.
A great movie.