And why do I keep getting conflicting reports???
Because people like bending reality to the shadow it casts on their mind. Right now there's a good amount of horse xbows on the EU battle servers. I started playing CRPG roughly a month ago and after a pole-arm build, I quickly made another character who went horse xbow. Why? I saw it mentioned that you could use a light xbow on horseback on the forums and the idea stuck. Little did I know that other people start treating you like you're using the AWP on horseback WHILE camping. Even if you're the first to charge forward, seeking out duels with lancers.
Now, a contingent of the Bandit clan is trying it out. There's a bunch of lads called 'US Cavalry' that do it while screaming 'LIBERTY, FREEDOM' and such phrases. My verdict on what role the HX has? You are there to zone out their lancers. If you can't kill them with a well-placed HS as you charge one another, just kill their horses. The high armor/hp horses might take a few bolts, but because you do high piercing damage they still go down. Their immobility lets you easily put in some headshots. As long as you stick to a lancer-buddy, you should be able to cause terror. Just flank 'em and they'll be too busy sticking to their defensive pose to be a threat.
Truthfully, I think horse archers are 'just as bad'. The difference is is that a HX has better aim but less shots, whereas a HA can spam away. Good HA's absolutely devastate any HX tho :/
tl;dr - people are starting to try it out on EU and some are becoming pretty good at it, inspiring more people to try it out.