Its not nearly 50%, but indeed, if GK is playing and we got some bad luck there are 20-30 cav on one team and like 5-10 on the other. These are only extremes that happen a few times a night at most but indeed can be annoying.
Archers/ xbowers: Shoot horses and don't wander off, look for high ground/ buildings to hide.
Polearmers: get a pike
Shielders: get something as simple as a spear on your back and you should be fine
Trowers: you are the ultimate cav killers
2 handers, just don't wander off by yourselves and you are fine...
So i don't think its needed to get a new balancing system to even out the amount of horses on each team.