Sure, it evens out in the long term, which is months and years statistically.
It doesn't even out in the short term. A single difficulty can change in 7-10 days.
Right, but this also means you have a chance at an unusually high payoff.
The larger a pool is, the better it's protected against massive luck swings caused by raising difficulty level & variance.
Pool luck varies about 5-15% at the most on giants like Slush and deepbit.
On pools below 500 ghash, there are daily swings upwards of 20-50% or worse.
Right, but I don't want to be protected from massive good luck swings.
Smaller pools have more variance, that's true. That's good and its bad.