This is more a case of "among the blind the one-eyed is king" rather than a testement to the usefulness of cavalry (blind = infantry).I find melee cavalry is actually useful, to counter horsearchers and archers, for example. I have to admit that it strains the cavalry a lot in heavy fights (high casualty rate) and that they need a backup from a solid shieldwall and from archers. I had a battle lately with a 130 stack (25 cavalry, 35 archers 75 melee infantry (about 30% of the infantry were T1 or T2, but all T1 in my game have shields; as I recruit mostly from prisoners, the cavalry were a wild mix of high tier, cataphracts, banner knights, vanguards, faris, some T6 from Customs Spawns and some unarmored T4 from my modded tribal troop tree) against Khuzaits with about 165 troops with 55 horsearchers and 30 mostly heavy melee cavalry, and they wrecked me several times (I reloaded) till I attacked the horsearchers early and dispersed them and the other cavalry in a chaotic horse fight. I had only 5 cavalry left after the victory or so. I use RBM AI module however, so maybe it may differ in vanilla.
To the noble spawn, it should be toned town considerably, it is not immersive to have so many of them. I hope after release there will be also mods which offer a certain max percentage division of the party, to avoid all elite armies.
That said, cavalry, in limited numbers, can have a use as support for foot archers; it mainly depends on where you are fighting/which is easier to replace.