That "imperfect seal" around the O2 mask yarn is rubbish - we disproved it at Lockheed in the eighties when a much-needed instrumentation engineer refused to part with his full beard. We put him in a USAF altitude chamber and there was absolutely no truth to that myth.
The REAL reason they are dangerous, though, is that they can become saturated with pure O2 just before you yank off the mask and run through a fire to ground egress. If so - why do we keep the polyester uniforms, especially ties? A ready-made face torch.
Too bad beards are universally condemned here in US airlines - I had a good one when I flew corporate. It broke up the monotony of my face.....