It's to stop a specific type of pool-hopping attack. If you look closer it says that the share payout method described is a simplification, with a link to slush's post about why, and how.
It's really annoying because I actually use my computer for playing games, but I don't know of any other pools that aren't deepbit (I don't want to contribute to the 51% problem) or don't seem dodgy.
Probably a bit late to start even a private pool if I don't have any ASICs