Well, my humble opinion is that melee cav should be anti ranged infantry. Sword cav could overall better while spear cav won't be so useful against buildings, other cav, trade, but better against ranged infantry. Maybe they need both better armour against pierce.
Tomorrow I would make some test, but seems that aren't cost-effective against ranged infantry. Killing rams and trade seems a very little role to a unit that I think that should have more presence in the battlefield.