You know...I think Kakashi wasn't playing stupid...you know; Raikiri would kill any shinobi on direct contact...I think Kakashi wanted to avoid any future mishaps by halving his chakra!
Anyway...I think I know why Tobi only reveals one if his eyes...it's just a theory; I also see it as the same reason why Danzou shows only one of his eyes...not that the EMS is for one eye though.
That's all I could put together...even though I know my theories always come out false.
EDIT: It seems pain is just a big sucker...he seems to be only good at mysterious stuff like summons and genjutsu...no cool skills.