The difference between Google and Bing is very simple: MSN has already been slapped with anti-trust. They haven't engaged in predatory practices since AFAIK. They've behaved well, and have a continued incentive to behave well, because another anti-trust suit might be a lot harder on them.
Google, OTOH, has gotten away with it so far. And, as Reno mentioned, parts of Google certainly have a cozy relationship with the govt that is worrying.
In any case, I don't think anyone's advocating for Bing to replace Google in terms of search market share. I'd just like to see them split evenly on search - each of them owning maybe 30-40%, with the other engines splitting what's left.