When is the last time a US soldier had to actually use a gas mask for anything but training?
Iraq
We personally had to use them in 1990 in Kuwait. Or detectors kept picking up nerve agents.
The unit I would later join, 1-34 Armor, used their gas masks is Desert Shield. It was a false alarm, but they did the whole drill - put on masks, did wipe tests, had the lowest ranking guy test the waters and take off his mask, etc.
You may remember that the threat of chemical warfare was a big part of Desert Shield/Storm....and civilians in Israel were masking up whenever a SCUD hit.
The mask is a legitimate concern. Within the last year, Syria has been accused of using gas - its not like its no longer used.
In the US Navy, where firefighting is needed within enclosed spaces, special firefighting equipment is needed--equipment that fits over the face, and supplies oxygen to the firefighter. I think a thick, heavy beard would not be well-suited for such PPE.
I've been a first responder, you don't get 30 minutes to an hour to cut and dry shave in the middle of a hazmat response.
IIRC, troops deployed with MOP gear in both gulf wars, and had to drill in it too. Having this policy then would have involved a lot of bitching and potential reassignments. What happens when your critical comm specialist needs to be reassigned because he doesn't want to shave?
There's a LOT of crap our warriors prepare for and subsequently don't encounter. The purpose is so when you DO encounter it for the first time, it isn't a battle loser.