Eligius does it too right? They don't guarantee it 100% of the time but most payments are made that way.
EDIT: To clarify how this works, the miners who contribute to mining a block don't necessarily get paid from that block, their shares get credited like any other pool and upon reaching a minimum payment threshold their eventual payments are made using some future block's coinbase. That avoids the issues p2pool has with dust payouts vs. minimum share value.
Right - that works for big pools that find blocks often. If you aren't a huge pool then payment variance (or just payment waiting period) would not make miners happy :p
Well as I said even eligius doesn't guarantee all payouts to be done that way. You could just make a regular payout if you go too long in between blocks.
Of course if the pool isn't finding blocks then you aren't going to be making payouts period (unless PPS).