If they'd actually done something with the short lifespan, it would have been interesting.
I agree that the episodes writtern for Kes were weak. They were too focused on her being a repressed entity blossoming into new, unknown powers, namely her psychic ability. Before and After, though, was a strong episode for her.
That said, Kes' presence on the ship, due in no small part to the actress, did a lot to cement together the social life of the crew. Obviously, the Maquis and Starfleet crews melded together in a somewhat miraculous and underanalyzed manner. However, Kes brought specific characters together, whom otherwise might have never learned to trust one another: Paris, Tuvok and Neelix. Moreover, the Doctor was largely prodded by Kes to explore his existence beyond being a computer program. It's impossible to see the Doctor's development without her, and no other character could have credibly pushed the Doctor in the way Kes did (certainly not Seven of Nine).
Kes' influence was the spark for a lot of the development that did occur on the show.