Yes, they use them because the bacteria grow more quickly in an oxygen rich environment. In an aquarium, it is on everything even the glass but they grow more slowly. So even if you’re not using a media, you will still have biological growth. Antibiotics would help control the growth (and even some algae), but I would think that would get expensive in a Water Tower. The trick is to find something that won’t degrade the materials, get all over the place, and perform biological growth control. I don’t know what that would be yet, but I will experiment when I get running.
Until that is found, I guess it is a trade off. You will get better cooling because the water is moving down the tower slower and spread over more area (plus you lose much of the peeing sound). The down side is that you will have to clean it more often.
I saw something else tonight that may perform somewhere in-between Bio-Balls and Pin-Pong balls. I was at Wall-Mart and they had a bag of practice golf balls for about $4. They are hollow plastic that have holes drilled all over.