The proposed reward system is to give 18.75 BTC to the finder who then gets no credit for the work the finder contributed to the pool for that block. That leaves 6.25 BTC to split between me and the other miners. Now, work_i_did / (work_pool_did - work_finder_did) is still very close to 0.04%, which means I could expect a payout not much over 0.0025 BTC each round.
True, but it is still perfectly fair, since if you find a block, you get to keep most of the reward. So it's only fair that if someone else finds a block, they get to keep most of the reward.
Such a pool could give each miner a slider from 0% (which is what normal pool mining is) to 100% (which is what solo mining is). If you set your slider to X%, that means X% is deducted from every share you earn, but you get to keep X% of every block you mine.
Why pool mine then? You can simply solomine and get 100% of the block reward.
The advantage for people who set it to 100% is that they get to use the pool's administrative functions such as monitoring their hash rate, emailing them if their miner stops, a nice GUI, and so on. They also don't have to bother running and updating a bitcoin client. But it would also give you a nice way to transition between pool mining, solo mining, and everything inbetween.