In a bedroom scenario with lube handy, sure spit is useless, but those spur the moment fucks in random places you have to improvise a little sometimes.
During missionary I actually find it arousing to have a girl bites my chest and arms while going hard at it, but I'm a biter in return... ;). I love to bite nipples and from the neck up. Of course nibbling on the clit while using my fingers turns not only her but myself on as well.
During a hot blowjob when a girl goes deep throat on me and then backs up and spits all over my dick I find it extremely naughty and a turn on. I wouldn't spit on a girls sweet lips, but licking my fingers before inserting them inside isn't all that bad is it? My spouse completely hates saliva in the bedroom. Still haven't been successful in teaching her not to swallow while giving me oral, the sloppiness is a turn on and let's me know I'm more than wet enough to enter her slit.