Yeah, certain cav units and sometimes in specific conditions. Feudal Knights dismounting into Feudal Foot Knights, during seiges only, comes to mind. (Why only during seiges, I'll never know).

I wish there were a similar option in RTW. Unless I'm a defender, I'm not finding much use for my cavalry during seiges. Since most of the family member's bodyguards are cavalry, it'd be nice to be able to get them off their horses and slog it out like real men.

Besides, wasn't it said Alec the Great himself was often climbing ladders with his men during seiges?