If a squad can drop their seige equipment, or leave their artillery sitting in the field, they should be able to hop off a horse. Perhaps it does open a whole new can of worms, having to deal with mounting and dismounting in battle (what happens to the horses? what happens if a unit has more men than horses? or more horses than men?), but I agree that all horsemen ought to be able to get off of their horses if necessary.
Perhaps it would unbalance things?