Gas didn't come into play until WWI, so beards were NBD. But I'm with Roo_ster. I'm not waxing my beard while trying to put on a Pro-mask.
Same with short hair, because of Lice.
Plus you do want everyone to look uniform, things that stand out get shot at. That why Aircraft Carriers are called bomb magnets.
I wouldn't be a fan of beards in Garrison. SSG and above maybe, but a bunch of 18-20 year old E1-4's with patchy, fuzzy teanage "beards"?
Ummmm, no.