The episode was decent except for the ending. Why couldn't those beings just take his power away and give him back to Kirk as a regular human being?
Because they couldn't.
Of course, it's an arbitrary device to give the story a downbeat ending. But I'm reminded of Roger Ebert's comment on the harrowing speeder-bike chase through the dense forest in Star Wars: Return of the Jedi
. He wondered why the bikes couldn't just fly above the treetops. Answer: Because That's Not How They Work!
(Come to think of it, maybe it was Gene Siskel who said that. I'm not sure.)