Man, the new guy that just brought almost 12MH/s online killed my earnings per block.
In theory when people do that we solve enough additional blocks for the payout to stay the same over a given time period. There's still some luck involved, but the more hashing the pool gets, the more stable the payouts should be. So I guess the more the merrier.

At least until the server falls over. heh.
That was my understanding, but it's just kind of demoralizing seeing that number drop by so much in such a short amount of time.