From a game theory perspective:
I don't see it often mentioned, but cav are tanks (horse HP/armor, can park in front of people and be a meatwall), DPS (they have one-shot damage potential at speed, stab damage throughout a match, this is very high against bad players and on pub servers), opportunists (spawn camp, mobility to finish weak targets, ability to turn separated 1v1 or 2v2 into their teams favor, able to respawn the fastest, flanks), they're also the ONLY support class with hard CC with knockdowns, can bodyblock, and can capture flags.
All this to say, if cav ended up bottom frags in almost every game they played in, cav would still be 'good' due to the extreme number of additional tasks and unique value they can provide. In fact, it would be expected that this class have the least damage of all the classes (although a decent final blow count due to chase/opportunist potential).