ME DURRHURR ME DURRHURR ME DURRHURR
No seriously, Khergit tribesman can have horses, but not all do, so they give you cavalry AND infantry in one class. also, like the nords, they have varied equipment- anything from javelins to battleaxes.
Nords grow up to be ungodly Huscarls, which imo are simply the best single unit to have in a siege. But on the battlefield, Khergits have something no other race has- mounted archers, and very effective ones at that. they also have decent infantry and Khergit Lancers in their evolution tree. I usually run mostly cavalry, and i couldn't live without my Khergit Veteran Horse Archers.