Well there are several keepers that have their Pacs on just water. One serious problem that can occur is that they can lose the use of their legs because they have nothing to grip and if the try to burrow or hop they slip on the bottom of the tank. This can weaken the muscles in their legs and cause them to eventually lose the use of their legs. Not something that you want to happen.

Not only this, but they don't feel secure in a tank with just water with nowhere to hide. This is stressful and stress is the number one killer of CB frogs.