For any given difficulty level, it doesn’t even matter how many other machines are generating.
You are true for long term. But I think many people with poor CPUs simply switch off their minings because they see it will take ages until they receive single bitcoin from that. When you are in cluster of cooperative miners, you will receive small amount (say 0.01 BTC if you contribute 0.02% to cluster performance) but more often. I feel that it may be a motivation for people to not shut down their miners.
I think it is extremely important for bitcoin economy to diversify mining across whole network and not leave mining on few lucky guys with fast GPUs.
ribuckThe only thing that counts significantly is the difficulty level and your khash/s.
Of course. The more khash/s in cluster, the more blocks found by cluster and more often payments to cluster members.