Originally Posted by Negthareas
If they are mercs, than you should not have control over them - it is a risk, but there could be the possibility of payback for a percentage of units lost.
Otherwise, how is it different than fighting with an allied player against an enemy, etc.?
the enemy does not know who is helping, (hopefully) your units can give some heavy help with another player fighting together with the main player. However that should really depend, because if there is a certain payback for each unit lost then what kinda mercenaries would you guys hire? I would take the archers, long ranged and less losses. You get more for the money