In the final analysis we must all confess that you cannot be neutral about Tau. Either you like or dislike them, thus they either are evil or not. This is why
GW will never fill in the blank completely--same with
DE & Necrons. They want you to think their must be some redeeming quality somewhere burried, but in fact they are merely targets for Space Marine adventures.
As a leap of faith, I've decided that Tau are the "least" evil of the xenos, and about as "good" as the
IG.
What's not to love? Fancy robotic suits, stealth tech, myopic fish, faced, blue skinned commies.
Join or die...oh yeah we have food. Of course they used native levies. As the British empire did once, the Tau live by the maxim "fight to the last ally"