No idea. I mean, I know what I would do, if I were them: use the mind control glands to make the Tau worship the Eldar. But that doesn't seem to be the plan.

Given their power of precognition, I suppose they are building up the Tau to hold off some future threat to the Galaxy. Using them to counter chaos or necrons, most likely. (Both theories have some likelihood to them: Tau are relatively resistant to corruption, making them rather useful against chaos. And there is a theory floating around that they are actually re-built Necrontyr, sharing the lack of warp signature and the short lifespan on a brutal world).