The word on the street is that public pools are under block withholding attacks. Malice it is.
Is this the same street that says Elvis is still alive? It's still all rumors at this point.
If you have a miner that CAN'T solve a block, then you need to point it at a pool to at least get some return, even though you're bringing everyone's earnings down including your own.
If you have a miner that CAN solve a block I still can't see a scenario where it makes any kind of economic sense to stage an attack of this kind.
If you have a miner that COULD solve a block but ISN'T and you don't know why, then you'd put it into a pool. If and when you figure it out and fix it you'll pull it from the pool.
My pet theory based on my Magic 8 Ball is that it's the last of the three. Maybe. But I might change my mind tomorrow.