I say keep the throwing. It's a perfect ambush weapon especially since there are almost no throwers at all around. It also helps when you see mr.bigbad coming charging up the ladder or standing on top of it waiting and you want to soften him up first, especially if you have 3 ps and some wpf. You won't kill anyone but your gonna give yourself a huge advantage. It will also be good for battle when a horseman comes around and as you said. Archers who run away when you get close is really annoying and a wardart to the back is a great way to catch up with them
. I remember before the patch when I was defending I always took 3 stones with me which I could chug down the ladder while waiting for them to get close. Funny how many people are afraid of throwers and slow down or even
goes down again when you start flinching them. This was with 0 PT/1wpf btw.
3 Riding wont be any good at all and there's really no reason to use it if your playing siege.
The 3 points in shield might work very well for you. I have 3 shield with my current 18/18 cav and it has saved me more than once. But if you're gonna put them there then you might want to take one away from athl so you can have 4 shieldskill. Lots of nice shields then.
3 if is not that good btw. If you want IF then you should max it, at least that's my opinion. Better to have 3 shield and bring one with you. Altough you can take those old wpf in throwing and put them in xbow. 90wpf will let you hit things at a medium distance and when there are shieldwalls at long ranges you can most of the time manage to hit people behind the wall instead of hitting a shield or missing horribly as those with 1 wpf does. In siege the crossbow with 90 wpf is really good while your waiting for the enemy to climb, altough you can do the same with throwing as I've already said.