Two legit reasons have been given -- can't say pools do either or these (and these are just the ones I've heard of).
1) Pool op sells mining contracts. Pool op sells hashing power @ >115% value, collects arbitrage profit, lures miners with better rates.
Dont understand what you are saying here. Why would a pool operator sell his hashing power >115%? I can imagine pool ops buying
hash rate above market value to test software, or help kickstart a new pool, but those would be really really small markets. Ive seen Dr Haribo rent some hashing power to test the new backend of bitminter, but it only takes an hour or so to get results.
2) Pool op is unable to purchase Bitcoins using his native currency, but either wants BTC, or has people lining up to buy the BTC. Because others are also unable to buy BTC in their native currency, they pay a significant (>115% market value) premium.
This makes no sense. Its what the SA guy (clipse?) claims, but think about. He is paying miners with bitcoins. Not only that, he is paying them more
bitcoins that the hashrate can normally provide. How does that help obtain bitcoins? It doesnt, he has to buy or otherwise obtain more bitcoins to pay his miners, so its a net bitcoin loss. If he has problems buying bitcoins, buying hashrate this way only makes the problem bigger.
As for actual uses: pool hopping proportional pools like Deepbit is one likely candidate. Deepbit delays stats by an hour, but since they are so big, Im sure just looking at the blockchain would let you guess with reasonable confidence when they mined a block. I wouldnt be surprised if you could hop it.