Kes's story might not have been going anywhere fast, but she was still 10,000 times more interesting than that ensign guy (whose character went absolutely nowhere). Her interactions with the Doctor were so much better than the cheesy cheese duo of Paris and his young ward, and I really missed seeing her in sickbay for the rest of the show.
Besides that, Kes was a talented nurse on a terribly undermanned medical staff, and was developing telepathic powers that Troi couldn't even dream of. What did Kim do? Some undefined console work on the bridge, and undefined science/engineering filler. There was no reason to keep him in the show. Oh, but his actor ranked in a magazine article, clearly he was the better choice.