I think the point is...
You may be able to make a little more by mining @ a pps pool during the beginning of slush's rounds, then coming on at the back end of a round on slush to get your reward for that block.
BUT, you won't be taking extra profits from everyone else @ slush. You will still be getting your fair share. As opposed to mining only right at the beginning of rounds, when you find a very short round, you actually make more than you normally would, and take a percentage of the share from the other miners on slush.
I could be way off base here, but that's how I interpreted it...
So you are saying a miner working for an hour before a block is found is not actually doing anything towards finding the block?
You are contributing little work but getting a big share from the block. The block has the same value no matter how many miners are involved, so those who have been mining continually with slush have to lose out!
That's one way of looking at it. the other way is... No one knows how long it's gonna be before we find a block. So I'm going to set my miners and forget them for the best average payout on all blocks.
Because you can say that miners aren't contributing for the first hour of a block, or you could say they are helping to find the block in less than a hour. It's all relative, math, averages, etc. Play the averages, it's your best bet.
You can use Automation and basic math strategies to employ that hypothetical exploit. It wouldn't be hard to implement that. Doing complex tasks based on stats is pretty easy with automation.