Whats wrong with a trackball?
You've never owned a blackberry before I guess. Or a mouse that wasn't optical.
Gunk (I.E. your dead skin bits) gets into it and can muck up the senors and make it un-usable, requiring you to remove it and clean it.
And if its a tale of a blackberry: Them just falling out.
Ohhh you have no idea how many issues I've had with the trackball (I have the BB Tour, which has the worst trackball of any BB). My thing is that what all could you possibly use it for? And I doubt you'd use it enough to get that much debris (except for maybe dust/lint) in it to have to clean it.
I just think that doesn't really make that big of a difference since you don't need it like 98% of the time.