Perhaps I am lacking skill (in form of knowledge) here, but I don't see any reason where jumping would help against cav. If you don't jump an strike horizontally just stay on the ground and aim a bit above. 
Or is it really about those centimeters when the tip of the lance is under your feet and the few centimeters you get horizontally if you don't aim above?
Jumping at the right moment will always make your movement harder to predict and therefore cav will have a harder chance to hit you properly. I see a lot of people jump swing cavalry and it's much easier to get a clear hit on the rider if they don't have a shield/are really gunning for the attack.
Part of that is personal experience, sure, I nearly always jump to avoid a single cavalry player and if I think I am in with a chance, I swing for them, I have quite often killed a rider when they've missed due to my large movement change and allowed me that opportunity to counter.
It's particularly useful for archers because of our low ps, if you maximise your movement, even if you land and can't move for a time, you've still launched yourself out of the way. Maybe it's a placebo effect but I feel you jump faster than you can move, particularly if you're not up to full speed.
And you seriously can't tell me you've never launched yourself off a high vantage point at an unsuspecting cavalryman and killed them outright... If you haven't, you're really really missing out on a big thrill! ^^
But on the point of realism here, i'm not sure how jumping would reduce your physical strength... Just because you're not grounded to get your lower body to push against the target and counter balance the force with which you struck your enemy, you'd still have most of your strength, you'd be leaning into the swing and adjusting your body weight to land it properly.
I can see where you're coming from though, you can't use your lower body to make sure you don't get knocked around by the force, but i'm sure you'd be able to do something to keep the damage as high. Nonetheless, Tenne no likey.
EDIT: Short weapons, harder to hit for the rider/higher if you're not wielding a reasonably sized weapon. For example, probably the reason why I disagree with you on this is that I use a langes messer, 92 length I believe and a polearm stab, not very useful against cavalry and I believe, having mistaken you several times as a Guard while you use our banned, you are a polearms character with a spear. Much more effective against cav!
