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...