Welcome to Librarium Online!
My upcoming blog in the works:
haha glad someone else's mind is in the gutter.
But seriously, in a war, the Eldar are like...LEx Luthor in SMallville, and the Tau are like Clark Kent, minus the super powers and loving mom/pops. And the sole survivors of a planet. All those things go to the Eldar, but you get the point. Occasionally they need something from the lesser party, and don't/can't obtain it by force. So they "persuade" you to do it.
Home-Brewed Codex: Farsight Enclaves
Well Tau will fight along side them if the Eldar join with the Tau Empire thats what it says in there army book at least.