Thumb rests allow the joysticks to hit the covers, giving the joystick more freedom of movement. If you don't use risers, you're fine. However, this causes a problem when using risers. I've tried 3 different pairs of control freaks and they all have the same problem. Please note that they are very smooth, almost like a powder coating on the lid. Strange feeling. Expected it to go away after use but still smooth after a week.