Again, I see where you are coming from, but since the guardians would always have to be combat ready, there wouldn't be very much leisure time, thus making shaving a burden. The guardian would have to run around buying razors or trimmers or whatever, then find a place where he/she/it would then shave their face consuming a large amount of time. I believe that the helmet would provide a sort of automated system in which it would control the beard placement keeping it well and straight and under control, after all, it is the future and grooming would also advance among other technologies.