spit is fantastic lube. Doesn't dry you out, lubricates well, isn't overly sticky, mixes well with your body's other natural lubricants. In addition it's pretty much odorless, tasteless, flavorless (unless you've got some problems going on in your mouth). cheap, readily available, sexier than pulling out a bottle of KY. lots of benefits to spit.

I'd also say it's nowhere near as bad as the watersports/scat stuff. That's a whole different game.