Priority targets, from biggest threat to the least are: Archers, crossbowmen, cavalry, Pole/ 2 handers, shielders.
When I aim and shoot, I have circa 1-5 sec to fire my bolt and get the hell out of dodge. Otherwise, some bugger just like me is gonna pin me.
But the absolute biggest reason to shoot other ranged are:
They are standing still.
Melee move erratic, and when the reloading is as long as it is, I want to make that shot count. Why miss a melee when you can kill an archer?
I love shooting 2-handers and shielders, especially when they have lots of armour and are chopping up my team (I always target my fellow mercs). In most cases it's better to "safe" the shot then to risk it.