when you say pirate eldar faction, what exactly are you talking bout? corsairs? outcasts? them dark eldar...
I cannot think of any reason a group of Corsairs would risk themselves in this war unless paid by the Craftworlds. What would they have to gain? It's not like they're going to risk their ships and plunder for a few Spirit Stones.
Explore, rape, pillage, folk-dances - all the same for them, really. As long as they don't feel constrained, they'll do whatever they want.
