I must admit that I am not a fan of beards, but then some of my mates have them due to their religious beliefs, so it is hard to say ‘no beards’ when they get to have one due to their imaginary friends. So, it’s either all or nothing, so I think it is indefensible to say no to some and yes to others.
Here is an interesting scientific study into firefighter’s masks and the effects of beards on their performance:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564146/
Now, what is often interesting whenever your life depends on a good seal - fire fighting, needing pressure or 100% oxygen breathing or in CBRN then it is amazing how quickly the razors come out!!! If I held the risk of Aircrew flying with beards, where even some of the masks have stamped inside them that beards will lessen their effectiveness, then there is no way, coupled to studies like the above, that I would authorise beards for Aircrew flying in aircraft with oxygen systems.