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TL;DR PPS incentives the pool to operate at max efficiency, which is good for the miners and good for the network.
There is no such "max efficiency" related to PPS.
PPS means zero variance.
PPLNS variance depends on the pool size. If the pool is large, then it's irrelevant.
PPS means higher fees.
PPLNS means lower fees.
The only thing a PPS pool can do,
which this pool does not do, is make sure they keep enough BTC in their wallet and charge a high enough fee, to handle a downturn in luck.
PPLNS doesn't have to do this.
Again, I've outlined here, using Meni's statistical analysis of pools, how you can determine how much BTC you
need and what fee you
need to charge for PPS:
https://bitcointalk.org/index.php?topic=104664.msg48795003#msg48795003