Well lets go through all of season four, and list all the "Harry" episodes, and of all those "Harry" episodes, how many of them could Kes have subbed in for without any friction?
They fgure out that he's the most popular actor on the cast, destroy a little blond girls sense of well being and then they only award him 1 out of 26 episodes to showcase his amazing celebrity?
Thinking harder, I don't think Demon was really a singularly Harry Episode.
He gets 3 episodes in season 5, Nothing in season 6 and 1 episode in season 7.
It's almost like Berman resented writing for him.