Oh my, these arguments like "Have you ever tried it? blah blah", honestly, just go to the circuss, watch what they can do on horsebacks, then think about it a bit, the logical conclusion you should come to is that with enough training there would be no problem whatsoever to draw an xbow on horseback.
If you think horse xbower is more dangerous than HA, you haven't met good HA. With xbow, you really wanna hit every shot, and unless you're really good, you won't. The fact you have to reload, wich takes couple of seconds, and the fact that even a jump will interrupt this action, makes horse xbowman inferior to HA, who can just shoot all the time.
Also, while on foot the advantage of holding your aim indefinitly with xbow is a definite plus, on horse it doesnt really matter, you still usually shoot right after the reticule settles.
All in all, saying horse xbowman is better than HA is crap. HA will be always better, thanks to the rate of fire, that can't be interrupted.
Guess that's a bit off topic though ^^. I don't really care for light xbow, with enough wpf, the reload speed is damn fast, so it would make no sense to make it possible while moving. It would only encourage people to use it with no wpf, which would lead to crying on forums and nerfs. So no, don't do it ^^