If used right I think the show would have been better for it. But then there's the obvious point she probably wouldn't have been used right.
I think it's pretty clear they were setting up for a "Power Corrupts!" arc for her. Assuming it wasn't done as whimsically and fly-by-nightish as "Fury" it probably could have worked.
Maybe if they worked it such that Seven was immune to her powers, they could have set up a Tudor-esque conflict between Janeway's "daughters;" it might have had interesting potential.
I have to disagree here. I never saw any "power corrupt" scenario. On the contrary, Kes did show that she was strong-willed enough to withstand severe temptation in the long run.
Anyway, I think the show lost a lot with Kes, a good character who could have become conselour or at least a more important character in the long run.
Harry was the weakest character but I wish that he could have stayed as well. No one should have been kicked out, even if they had to add Seven.
As for the "Janeway's daughters" conflict, Kes was too smart to be dragged into a conflict. She would have used her skills fo find a peaceful solution, she was good at that.