The difference with deepbit versus a single entity (government, rich prince, etc) is that deepbit does not own it's computing power. It is at the mercy of those that give it to it. And, people are a fickle bunch. If anyone even slightly suspected that deepbit was up to no good, that could adversely effect any evil plot it's operate may or may not have.

And... as far as "since it would be impossible to catch him doing it... i think people would have to assume that he is"... there is a sentiment of supporting other pools just to keep things balanced.