FWIW, I wear one all the time, and use it pretty often. Aside from the fact that it's required in Laguna Beach, occasionally we'll make very long surface swims. And after 1/4 mile of swimming on your back, it feels better to flip over and use different muscles for a while, swimming face down with a snorkel. And I'm sure it's more efficient; I go much faster face down than on my back (due to the drag of the tanks).
Not sure why Laguna Beach requires it, other than the contingency where you don't want to or can't use your air supply and need to surface swim in choppy surf which, if on your back, might splash over your face and into your mouth/nose.
Personally, I feel appreciative when a Laguna lifeguard stops me to make sure I have a snorkel and a buddy. They've even given me the once over to make sure my gear is okay. Sure, I'm a big boy and can take care of myself, but I'll bet most folks who have had diving accidents would claim the same thing. You can never be too safe.