What's the point of comparing if your gonna forget anyway?

I think this is a combination of despair and the realization that your past self wouldn't die. It's very clear in Seiichi, he just thinks: "Fuck it, if I'm going to die might as well have fun". Not sure about the girls, but certainly possible.
It does show the biggest demerit, in that experience is virtually lost. Experience also cannot be shared. Teleporting people randomly just for registration is downright awful for morale. Though teens being teens, it was probably an exciting prospect.