Yeah and then what happens when you get gassed? If you're lucky you get 10 seconds heads-up to put on your mask and make sure it seals, and if you fuck it up well nice knowing you. A good seal is all the more important with a negative pressure system like a gas mask. SCBA and PAPR are at least positive pressure so there's an airflow gradient away from your nose and mouth in case there's a break in the seal. Chlorine, mustard, and sarin are awful ways to go: and all of 'em have been used recently in Syria (and a few decades ago in Iraq).

Secondly, growing a beard when you're trying to blend into the local population in a Muslim country is another use case, but it's a calculated risk based on intended diplomatic aims versus risk of a CBRN attack.