A less flexible thumb needs more bevel and/or more reverse if you want to get the hand/palm on the ball, creating more/better roll.
Your ball should not hang on your thumb when left at your side. It should fall off. If it doesn't, you will hang in the ball on most shots. You want to be able to release the ball when you want to.
You should need a slight(and I'm mean slight) grip with the thumb to hang on the ball. When you relax the thumb, the ball should fall/come off, when you want it to.
This way, you control the ball and not the ball controlling you. Also, it allows you to get out of the ball cleanly with not so perfect timing. Remember, nobody has perfect timing every shot. =:^D