My understanding is exactly how charles vail put it. And the old "scrape gum off the bottom of your shoe" technique (that you do with clipless) only hurt the knees.

In MTB clipless is pretty darn helpful to keep your feet stable when you're being tossed around. It also helps climbing, as if you don't make it up the hill, and you stall and have trouble clipping out, you will endure at least several moments of intense pain. It can be a real motivator.

It sounds backwards, but when I got used to clipless on the trails, using them on the roads was pretty easy.

In any case, you look like a real wuss without them. And you have no cajones.