Okay, so first let me apologize, as I am well aware that this idea is neither original, nor particularly novel, but while I was putzing around at work, hoping for it to end for the morning, I had this image of a movie clip, where we pan around the bridge of a Tau command ship, while they prepare to head out for a place where battle will occur, and as they are talking in small groups, we pan over to a group near the center. One is obviously an Ethereal, another a Water Caste Diplomat or Ambassador (Por'El or Por'O), and the last of the Tau is the Admiral (Kor'O; wonder what they look like on-bridge? Wearing uniform? Some battlesuit? A containment tank?) The last one, however, is certainly not a Tau, at all, but rather a Farseer, giving them tactical advice based on things she has seen upon casting the runes.
The Eldar are a powerful, ancient race, but their golden days are behind them; their armies are depleted, their resources withered, and their attention spread among numerous bizarre things that no non-Eldar would likely understand, anyway. Then add the Tau. Somehow, the young race has amazing tech, the capacity to mass-produce it, a rather formidable, standing army, and more. Their focus on the Greater Good should be a strong protection against the kinds of things that caused the Eldar to fall, and that they fear Humanity will emulate. Their martial focus is almost comparable to that of Aspect Warriors, they are resistant to the warp, in small degrees, and on. The Tau could benefit greatly from the accumulated knowledge of the Eldar, and the Webway could help solve their lack of warp travel; even the Webway must be faster than "skipping" along the border, and the mindset of the Tau would seem to offer some resistance to the degradation effects of long-term Webway exposure. The Eldar could benefit from the decent quantities of fighting force, and possibly even find a rather decent "successor race," those who haven't given up on living, in favor of the Ynnead Plan, anyway. A Craftworld traveling under the protection of a joint Eldar and Tau fleet could be terrifying.
So yeah, the idea has been broached numerous times. I'm bringing it up here, mostly, because DW is the aspect of these games where the Tau have a strong, organized presence, as opposed to RT, DH, or what have you. It could do a lot for the Tau, often categorized as the weakest faction in the Reach, to gain the assistance of the Eldar. So, did I miss any cool advantages? What might be some legit tripping points, beyond "the Eldar are arrogant douchebags, who don't believe they need any help!", which is possible, but wrong, because they do. Has anyone in a DW game shown it as attempted? I see the Tau as having prodigious amounts to gain, and the Eldar also benefiting, but mostly having to accept that someone needs to inherit their coolness, in case their other plans fail.
I also imagine Ebongrave, were he to hear of it, crapping his pants clear off, and them rocketing all the way back to the Gate, and then needing to tell his subordinate to bring him another pair of brown pants; it's going to be one of those days.