Horses are for me big, running banks of points.
Ranged are shooting horses because thay are their really dangerous oponents, and also it's helping your team to win.
I sometimes can deal with 1 cav guy in open field, but 2 are just deadly (especially 1h cav).
Heavy cav is not OP, they are paying hi upkeep for those heavy horses, but they are also quite easy to shoot down.
It's also true, that it's faster to kill rider than horse (on strat battles many times 1 horse could survive 2 or 3 riders
)