I just hold the view that to get the maximum out of your product allowing end-users to participate in its growth would be a wiser course of action.
I can see where you're coming from because I'm a gamer and I used to be a game developer as well, but there's a difference between what you're suggesting and what is right for this type of product. Jim's fears have always been that if he allowed end-users to modify his product, it might lead to exploding fish, or art that was far from tasteful. If that ever happened it might reflect back on his work and his reputation as an artist.
When you liken the screen savers to games (especially games like Quake, Unreal, etc), you're talking about games that focus on violence and so far the majority of the people that have made the same request that you have also happen to be gamers that play those same games. I'd probably be hesitant myself (were I in Jim's shoes) if FPS gamers made the same kind of request to me simply because of the type of content that might make it's way into the product (less than serene species, violent and aggressive AI behavior, alien fish, etc).
On the other hand, IF Jim and Prolific were to allow users to submit their own fish (model w/art) to Jim and Prolific for approval, that's another story altogether because that would allow them to control the content.