Despite the druidic ties between night elves and tauren currently, they are not very "buddy buddy". During the sundering, the night elves didn't even want to utilize tauren in the war against the demons and they regarded them as brutish beasts incapable of grasping the concept of civilized society.

That aside, I agree with your point that the Night Elves allying with the humans doesn't seem to have a strong motivator. They have always been a solitary race and they do not like to depend on other races at all. I'm sure the sundering knocked a lot of that arrogance out of them, but even after those events they've been withdrawn and prefer to keep to themselves.

I'd love to see a more in depth look at the Night Elves motivation to be a part of the Alliance. The other races that make it up I can understand, even if just from a unifying religious standpoint (via the Light).

Will be interesting to see the lore behind the Worgen, though. After all, when Forsaken regained their free will they tried to return to their homes and families and were chased away, which made them bitter. Yet the Worgen are monsters and affected by tainted magic and generally seem like something unacceptable that would be chased off, as well... get accepted with open arms? Really can't wait for Cata, if only for an answer to this! lol