The Tau force others to follow them because they believe they are working for the Greater Good, as you say. They believe that they and their client species will benefit in the end (or is it that everyone everywhere is supposed to benefit?) They seem to have relatively noble ideas. Their methods might be very flawed, but are they really as bad as the Imperium philosophy of "believe exactly what we believe, or we'll kill you"? Not to mention the fact that they're willing to form relationships with other species, whereas the Imperium just kill anything that isn't human, or at least the very loose definition of human in a universe where that term applies to this thing.
Also, I'd have though that in the bad situation humans were in, they'd want to form alliances with relatively agreeable races like the Tau or the Eldar, when the alternative is dealing with something like the tyranids.