I've been experimenting more with cavalry, not actual tests as in setting up battles with a mod, but just trying various things in game. For me anyway, it seems as though chaos is the key. Once the enemy is engaged with my foot troops or turning around trying to defend or attack horse archers, the melee cavalry seems to work much better.
I usually wait until the enemy is just getting into bow range and "F6" my horse archers (they don't even have to be Khan's Guards, any of them survive pretty well), have the infantry advance or charge, "F6" my melee cavalry, most times they will split up to protect the flanks and just sit there or go after enemy cavalry, and while this is going on my archers are getting shots into troops (some of which have their back turned trying to defend against the horse archers). About the time my infantry clashes with the enemy the melee cavalry will charge in to protect them (sometimes it requires manually getting them to group up again if they are tangled up with other cav units, then charging).
So cavalry isn't the old "fire and forget" unit that it was in the past (Warband), but they do seem to have their uses, even if it might be just keeping the enemy cavalry busy, they just seem to need some micro managing. Still, it is an expensive unit to upgrade and I seem to get stuck in quite a few "cavalry unfriendly" maps, so I usually only keep a small cavalry force.