
Originally Posted by
Clockwork-Knight
"As the Tau Empire expands out from its homeworld, the Tau inevitably encounter new races previously unknown to them, and to each of these an offer of allegiance is made. There are many aggressive, arrogant and selfish races in the galaxy, however, and even the Tau often find first contact results in nothing more than yet another bloody war. There are other races however, who readily accept the message of the greater good and take up their place in the Tau Empire. Some of these races are small, perhaps located on just a single world, or else primitive with little useful resource to offer the Tau, in which case their accession to the Empire is simply a formality, with the benevolent Tau offering protection to these lesser races while they can expect little other than appreciation and friendship in return. Other additions to the Empire are advanced in themselves, and the union of two such cultures provides valuable new knowledge, technology and understanding for both parties. Such races, where able, fulfil their debt to the Tau Empire by a series of tithes which suit their own particular abilities. Able craftsmen, for instance, may be called upon to provide manufacturing capacity, while aggressive or warlike races will be obligated to provide troops to the armies of the Tau. There are other races still who do not wish to fully submit to the Empire, but who likewise have no wish for war with the Tau and will instead strike up armistices or treaties of neutrality, opening up lucrative new markets or providing new allegiances for mutual protection. Such races are also likely to hire themselves out as mercenaries to the Tau Empire when the opportunity arises. "
Battlefleet Gothic Armada supplement
Sometimes, the tau conquer, other times, they just help. Sometimes, the tau demand help from their allies and subjects, and sometimes, they come to their aid without expecting any help from their belliguered allies. If you don't want to be part of them but you're okay with them being your neighbour, they're going to let you in peace.
All in all, they're good.