I don't see the guard mode as black & white as TWFanatic. Although it is true that on 1 vs. 1 the one without guardmode will win, it is not because they fight somehow worse it is because the one without guard mode surrounds the one on guardmode. The right way to test this is to eliminate the possibility of envelopment. I tested this a while ago, and the results were, that without envelopment a unit in guard mode does not fight worse than one without.

So, it's all about formation. If you need your units to keep their formation, guardmode on. If you need them to push and envelope or your formation has been broken, guard mode off.