Let's take two simple scenarios:

- A pool with three people. Two of them have clusters of 5970s, one of them has an old Pentium IV. For blocks that are found before 10000 shares or so, that third person won't even have submitted one share, and thus will get nothing, not even a small fraction of a bitcoin.

- A pool with three people. Each of them has an old Pentium IV. Blocks will take a long time to be found, but most likely a lot of the members will have submitted at least one share.