I love giving it and I've had several Girlfriends over the years ask for it and even those who don't specifically ask for it will usually go along.
Why?
For a pile of reasons.
From my point of view, I love putting my cock to a woman's face and shooting cum all over her. It's visually exciting and there is an element of sluttiness and degradation. I love super horny uninhibited women.
From the female point of view, some like the visual, some like being humiliated. Sex can be quite erotic when there is an element of humiliation to it. It's not a personal humiliation, it's sexual humiliation and being sexually humiliated tends to reset the psychie (myself included). Many women like to be dominated and a man treating her like a sex object and cumming all over her face is definitely a form of male domination. So it can be exciting.
I think that the SPs who don't allow it don't allow it because they are very sensitive to being humiliated given their profession. Also, there is the issue of exposure to semen and yes, it can wreck makeup too.
As a strange aside, sometimes when an unknown woman is giving me a hard time for whatever reason. Say I'm in the bank and they are putting me through the red tape ringer and she's got attitude, I just picture her in my mind with a facial running down her face and it makes me grin.