Sigh...making the strongbow 2 slots is simply silly. It's a weak bow.
HA's are designed to take down enemy cavalry. It's a hell of a lot harder to shoot infantry, so enemy cavalry is the natural choice. Besides, it takes a pure dedicated HA build to actually sucessfully be HA. That's a lot of dedicated points that means you're pretty crap in any other capacity.
If cavalry goes off and charges on it's own (say towards afk players at spawn), then you are going to get shot down because HA will hunt you. If cav was sensible and supported your infantry, rather than being lone heroes, then it wouldn't happen. If you want to go off on your own and hunt people, you've got to deal with the consequences if you see HA around.
HA don't get points for shooting horses down. We do it because enemy cav are rather effective at reducing a teams chance to win...therefore it's our impreative to shoot down your horse (plus there's a sadistic pleasure in shooting down lancers horses...you can almost hear the screams of rage
). Unfortunately for you, horse archers are kinda there to be annoying. It takes a lot of dedication, arrows and skill to get a high amount of kills. So instead, it is far easier to simply be a nuisance and take out enemy horses because they are the ones doing a lot of killing. It's one of the reasons I support some kind of assist system.
Horses however, are easy to hit regardless. Making the strongbow 2 slots wouldn't make a difference to horses being shot down. 15 arrows (if bodkin). It takes a maximum of 4 arrows. Horses are big targets. You do the math.
Besides...because HA is such a dedicated build, you rarely see them as it is. There's no point changing a bow just to try and nerf a few guys who actually play it. When you do see them, maybe you should change tactics to increase your surviveability (rather than trying to charge off as cav on your own).