I think there could possibly be certain "missions" in SHTF, WROL, where SPEED would be king, and there may not be as bit of a need for armor that could slow you down. Also, in a partial situation, armor may be too noticable, and get you killed anyway, and you have to go, looking like a normal civvy.
Remember, guerrillas have to BLEND in with the environment, not stick out like a sore thumb .
I believe there is a time and a place for armor and then no armor. Just like there is a time and a place for OC and for CC.
Both have their advantages and disadvantage. Armor can slow you down, and make you stick out like a sore thumb, yet it can also save your life when SHTF. It all depends on the situation and the mission. With that said, train with your armor on, because it will make you faster, on those missions when you don't have your armor on.