Well, curiosity and the thrill of doing something plain gross (which I am sure is a feeling many of us appreciate) might just get people there eventually. But it is likely not very common - it requires one to overcome a bunch of deeply rooted taboos from your earliest years. However, not very common might just be reason enough for some to try it.
And then I will break a rule of not being the one to raise a warning voice; the practice as described in the beginning of this thread is in fact risky. You will have to be damn unlucky to be killed, but your will have to be damned lucky no to catch a staph infection. And while staph infections is often only a nuisance (e.g. pink eyes), it is far from unseen that they get quite nasty.
The obvious stems from fecal matters (however diluted) running down onto your face, entering your eyes or (worse) your nose/throat, which is very hard to avoid in the position described.
If I were ever to experiment with this kind of shit (sorry, couldn't resist) I would at the very least turn my face away - but I guess this would spoil the best part of the show (literally).
And for the curious; yes, it smells bad but surprisingly does not taste (that) bad. (But expect it to be an acquired taste).
"Burnt" is the description you can read when researching and I concur that description. When diluted as in an enema (no. 2 or 3) the taste becomes equally diluted and more managable, but I doubt you can resist the urge to spit for long.
To end on a positive note; play to your hearts desire - after all most of us walk unusual paths to pleasure - and many of us did only find them through curiosity. But in this particular case, be careful; keep it out of your face.