Slush has to add a small fee to every payment. If the threshold was lower, this would become a substantial part of the pool's running expenses. As Slush takes only a small cut for running the pool (2%) there has to be a limit, or some people would claim every tiny amount they earn, and the thing would be unsustainable.
When Slush would include the payout transactions into his own block, he'd be able to pay it without transaction fees, wouldn't he? Can this be a solution?
(Of course this does not solve the problem of Satoshi dust accumulating in the miner's wallet.)