The only way I can see it being considered unfair is if there's a belief that every share needs to actually earn something, no matter how far removed it is from a success, rather than just earn a equal chance at something. But if that is a strong belief, just go with Deepbit-style pay-per-share. Other hard-to-understand improvised blended models create gaming opportunities for miners or the pool operator.
Straight PPS is vulnerable to withholding attack due to infinite risk for the pool operator.
Not straight PPS; the proposal I made. For distinction let's call it Pay-Per-Last-N-Shares (PPLNS).
There's no risk to the pool operator, they only pay when there's a hit, and they always pay exactly the last N shares received. Earlier shares get nothing.
There's no increased or decreased return to any miner based on time of entry or exit, because payouts have nothing to do with past history or the length of the current round or the pool operator's reserves or whatever.
My initial intuition was N could be any number up to the average number of shares expected to yield a hit, but on further thought, I think N can be anything so long as it is constant. Exercise left to the reader.