High difficulty hurt profit or not?

[thinkgad]

Hi all ... this is my first post.

Many pools claim that high difficulty setting will not hurt your profit. Middlecoin even use 512 as the only option with the same statement. But IMHO under a high difficulty slow rigs may not be able to submit shares before the pool move on to next block ... and basically will be burning electricity for nothing. If they are working on low diff shares at least they will be able to submit something, am I right?