I grew a beard about 6-7 years ago. It was initially just because I went through a period of unemployment and couldn't be bothered shaving. Then I had to go to a job interview, and it was long enough that it can't just be shaved off because it fouls the razor, so I bought a cheap trimmer. After the trim, I didn't bother shaving... and haven't since. I keep mine quite short - about 5mm after trimming.
The thing is, you can scrape a blade over your face every 1-3 days (depending on how fast your stubble grows and how neat you need to look for work)... but why, when you can just trim it down every two weeks? Quicker, no cuts, no dry skin, etc. Mind you, if you're one of those men with patch beard coverage, growing one isn't such an option if you still want to look good.
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Ultimately, a lot of this comes down to personal grooming. And all else being equal I would be very skeptical that my 5-10mm beard has more harmful bacteria than the average household pet given I wash regularly.
Although of course humans and dogs aren't equal. What we might consider is that furry animals are evolved in ways to be quite bacterial-resistant, because it would a minor disaster if their fur were breeding grounds for their own ill health. Humans, however, being vastly less furry, may have far less natural resistance because they just don't generally need it.
We might also consider that over time a sort of bacterial equilibrium would develop in humans - we're absolutely covered in bacteria, and a lot of them are neutral or beneficial (e.g. killing off the "bad" ones). However, washing may be perversely sort of counter-productive by killing a lot of the normal skin and hair flora, thus meaning more problematic micro-organisms are more able to gain purchase in the short term.
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Most importantly, we also need to consider that this is media-friendly but lightweight, bargain basement science which should not be trusted much at all.
