You increase initiative in the cost of other more important attributes, which means that merc may be low on HP or melee defence. And you only have 9 action points every turn, no matter how high initiative your mercs have. Therefore, it will be very hard to do hit&run tactic.
By the way, the hostile units like necromancer and crossbower are always deep behind the frontline, so you need to move your archer forward to hit them. Hence you may have risk to expose your low HP archers infront of enemy’s heavy infantry and ranged attacks.
