Higher hash rate = more blocks per hour = more consistent payouts in a pool that isn't PPS.
If the difficulty was relatively 'constant' as it was prior to Jan 2013, over a few months it wouldn't matter if you were mining PPS or PPLNS or DGM. The variance would average out between the methods. But now the difficulty is going up 20% every 12 days. So lets say you mine with a small pool, like Eclipse, and it has a bad luck week. You just lost out of income at a difficulty level that is gone forever. Sure it may have a better luck week in the future, but that will be at a higher difficulty, lower income per hash rate.
Perhaps I am seeing this wrong, but in order to maximize your ROI in this rapidly rising hash rate environment, I feel you need to be on a pool that gives consistent even rewards with very little variance. The really big PPLNS pools (BTCGuild) do that as does 50BTC. No surprise they are in the top 3 pools.