[st.patrik]

Hmmm... I'm afraid I really don't like your suggestion Gandalf. The change you suggest would make sense for experienced archers more than for skirmish archers, but D3 has its own mechanism for depicting experience and unit type isn't it (you could argue for a few exceptions, such as daughter/mother/crone, but definitely not in this case). Those mounted archers are as green as a newly recruited militia!
As to the idea of the higher positioned viewpoint, yes it would give a bit better visibility, but then again few battlegrounds are perfectly flat, and archers (unmounted) generally would take the high ground for this very reason - enough that they may be higher up standing on their own two feet than mounted archers on their horse on the plain.

By contrast, I do rather like BigJMoney's suggestion. It seems somewhat realistic too: I would imagine that units not specifically designed to 'fire on the move' would need a couple of minutes to ready themselves, lay down their extra bale of arrows etc.
The missile troops more used to shooting on the go would be prepared and so wouldn't need this time. They could move into position, loose a round or two, and immediately move back. This type of behavior could be hard-coded into the 'mobile-archer' commands so that they work differently from reg. archers.
I think once they are backed against the end of the field they should probably auto-rout, and I think that 'mobile archer'-types should get a minus to their precision (or other archers should get a plus), because of the unstable mount beneath them and the fact that they are loosing a lot of shafts quickly.