I don't understand this. In the extreme, if everyone left a pool, why would a miner join it? They would get a block no faster than working on their own and they would pay to previous workers who had shares.
In the extreme, yes. But if the powerful miners leave after a few thousand shares, the less powerful miners may find it profitable to stay, because they will get a bigger share of the rest. The problem, for small miners, is that when you're up against people that have 1000x your computing power, they will find blocks before you even contributed a share, and you will gain nothing.