I think they do it to pad the especially...quiet videos, at least most of the time. I generally don't pay much attention to it, anyway, just the action and noise. They've gotta make noise, or its just...wrong, to me.
Silent sex, to me, usually means you're doin somethin wrong. That, or you don't want to wake up your parents/flatmates/neighbors/whatever.
...Actually, that latter bit actually sounds kinda sexy, but I still prefer noise!
