Don't think occasional share contributors would agree to that. Since each round of finding block has a random number of shares depending on sheer luck. Some rounds might get solved with fewer number of shares than others. Also the N for PPLNS would have to be changed each time the difficulty changes. It seems a better method for payment but only among loyal pool members as it would mean considering a constant number of shares for the payment and that would not fit fine with pool hoppers. I say go for it.