You want to be good at both ground and on horse, you have to take a "lesser" horse. You want to get a good horse you have to sacrifice a bit in either footfighting or horsefighting.
We already take a hit in wpf, stats and skill points, and crpg money.
You can rest assured, when my 1h/p-arm/shield guy gets knocked off his horse, he's going to have probably less ability points, defenitely less wpf, and less iron flesh, than the 'no horse 1 weapon build guy', waiting to smash his head in, as he slides off his horse...
So your statement of 'bieng good at both' is not true, its bieng average at both, + a horse which SHOULD be a terrifyingly affective asset on the battlefield. -Which is already reflected in their upkeep costs, especially the ones with horsebump viability.
But once on foot, unless your opponent has done something to multi-specialize himself, mathamatically according to the system, The hybrid actually has '
less than average' stats.
So no, your wrong about the whole 'good at both' thing, (which your using as the basis for your argument)
And no, making horses taking slots, will simply kill a multitude of horse classes.
This is not the way to do it, defenitely not.