Spit is a funny thing. It's a bit/a lot of a mood killer for me when a partner leans back and spits on their hand (please, god, just use lube), especially if it's really drooly....or actually, if I can see it at all.
There is something about being in the midst of some rough play....having my partner lean forward, and just spit on me - either on my bits before roughly penetrating me, or on my face following a slap..... it does something to me. The first time I ever had a partner spit on my face in the middle of some SM stuff (context, right, it's all about context), I remember feeling overwhelmingly intimate and connected to him and to the moment.
Long story short? Different strokes for different folks. And nope, I don't get why it's so prevalent in porn.