Planet Waves makes a similar product called the VariGrip. The rubber part on the bottom can be removed and placed on top of the buttons. That exposes a textured surface on the bottom. By reversing the grip, buttons down, you can use the textured surface to maintain your calluses if you are ever away from your instrument for any length of time.
I would also not recommend using the grip as shown in the first post. If you are trying to build hand strength for playing, only use your thumb on the bottom of the grip, not your palm.