Mutual benefits; Tauren weren't exactly doing too good in their fight against the Centaur.
That aside, my answer would be Gnomes and probably Orcs. Orcs just had such a small presence on the world, it's hard to believe they ever actually became anything more than a footnote in history at best. Gnomes, they just don't really bring anything of value that you couldn't get from Dwarves. Dwarves build great battle machines, they could easily have picked up the mantle of "quirky small race interested in tinkering" as a subset of their racial interests.