You managed to get Drassen without combat? Without combating patrols you mean?
Anyway, Patrols are loot, and the more your mercs get to shoot things, the more their marksmanship will go up (which is an incredibly important stat in my opinion), so feel free to just kill them all.
You are outnumbered... you fight in the open... the hit probability is in the gutter...
There is no strategic play involved... it is simply chance against your favour.
I don't see much to gain beside wasting ammo for some small (ineffective) marksmanship.
There are a few maps that are pretty open, sure, but don't forget trees and rocks.
But to each their own. I agree it's easier taking down patrols once you get your hands on a few mini-14s and stuff like that; I kinda like wasting ammo in these early fights though, since pistols are soon useless anyway. :)