Actually I find that Craftworld Eldar have more relation to Vulcans than Elves. Their way of life is based on the supression of the emotional drives that destroyed their old civilization. The Outcasts are mostly made up of Eldar that couldn't put up with such a ridged lifestyle, usually either getting dumped on some planet with ranger gear or taking up life as a Corsair, though it seems they might sometimes get intergrated into other societies. While they are described as fickle they are actually logical in any of the fluff bits I've seen it's simply that the way they see into the future largely involves looking forward based on certain actions performed now. Often they have no idea why things turn out so much better if they do a certain thing, but when considering the butterfly effect over centuries it's easy to imagine how things could get pretty crazy.
I don't know if I buy into the idea that it would be a wise move for them to fully ally with the Tau. While the Tau sometimes seem like the "good guys" of 40K, they are as ruthlessly expansionistic as any of the other races that don't just get wiped out. The only way they're "better" is that they allow conqured races to survive as subjects rather than ethnic cleansing them.
I think the Eldars only real hope, given what I know of the setting, is to play everyone off each other long enough to turn around and build up their population and/or jumpstart their new god thing.
I actually play them on the tabletop. I was drawn to them because they're...hmmm how to phrase this, an army of wildly varying tactics and option.