That would make a really good addiction to Oblivion.
I mean a mod were you could damage enemy with the horse's strike, and it would gain strike level... So they could even attack other horse, like medieval duels.
Mounted combat would have been wierd,buggy and unbalancing. It would have been to heavily impacting to most combat (giving an advantage to mounted opponents versus grounded ones) and mostly detracting from the new grounded combat system they put so much hard work and energy on.