"Each to their own. You're obviously not a PC gamer, or if you are a very casual one."
Sorry, but a thumb based trackball is just as good as, if not better than, most mice.
For me it comes down to a few things:
1) The fact that you can get greater precision with your thumb than yo can with your wrist/arm.
2) You never run out of movement space. Sure, with a high sensitivity you will not often have to pick up your mouse and move it, but it does happen.
I set my sensitivity such that I can do a 180 turn in either direction with out lifting my thumb. This is a fairly low sensitivity, which means I have (for me) rather fine control when I want it, however I can also do big movements when needed.
The one thing that keeps tempting me to get a regular mouse is more buttons, and what I really want is a speedpad type thing instead Something that gives me movement keys + lots of extra buttons for my left hand). The one I liked was the steelseries Fang, however it isn't being made anymore...