I usually dont have light cav in an army, if they are there most likely it was a free unit for a mission or the like. Having a unit that is only designed to go fast
and chase stuff isnt really what Im looking for. I suppose its the type of armies you build. I usually build heavy infantry or heavy missle so the cavs ill have will be heavy always (mailed at least) and few in number (1-3 max). Also when I send cav I do not expect any to return. (fire and forget) This is why my fav cav is probably a cheap merc unit, they can chase the archers into skirmish just as easy and when they rout after you send them on a round about attack to a treb or something, you will feel no loss. As I said b4, in mtw2 any cav army I make, I expect for them to A) all die and B) to cause an equal or greater loss to the enemy.

For wall sieges usually a heavy infantry for the 1st up the wall, after that I send militias or cheap subs. Militia is very useful for being the 1st wave... put a town militia on loose formation and send it running right in front of a ram for example... they will absorb most of the tower and arrow fire allowing the ram to make it there.

If you only have militia or early units to siege with then make sure to build alot
of ladders and towers to spread your attack out along the walls. The wall defense bonus gets lost quick when the unit is surrounded.