That’s the thing though, going without a helmet into combat is just about the worst mistake you can make. Not having a helmet is something for movies, as you’d just die without one in a fight.
Toggling them on and off would also conflict with the idea of everything that you see on the map actually being there and would remove an important visual short-hand for what state your merc is in.
However, If you need help identifying your merc, how about having a portrait of the unit’s head in the command UI on the bottom, in addition to the name? It would need to be unobtrusive, so I don’t have a mental image of how you’d make it work without conflicting with the design, but it’s something that the future UI programmer might be able to pull off.
I agree with you that going without a helmet would probably result in quick death in combat. However, I am unsure whether actual medieval helmets offered that much protection against blows, and if their defensive value actually outweighed the costs of wearing them, but that is a question that leads to endless debate and most likely not to fun gameplay :)
I do still feel that identifying mercs is more difficult than it should be. Maybe the subject should be left to the eventual UI design, but it should not be forgotten. The unobstructed face in the command is actually a neat idea. It could appear right next to the name (which should definitely show the title, as the ID issue gets only worse when several mercs have the same name :) )