I wear one. I had trouble finding them, as well, so I fabricated one out of polycabonate. It's already saved my face a couple times from flung stones. I always had issues getting a full face on. Always seemed so cramped, as opposed to, say, a Motocross helmet, where the chinbar sticks out, more.
I also, since owning the aforementioned helmet setup, bought a modular helmet. No more cramped quarters when taking off and putting on. The visor is a plus for cold mornings, but it DOES get annoying when it fogs up, and I have to play the raise/lower game at lights.
Between the two, I have tons of flexibility for any riding occasion. There's no preconception that the chinbar will offer any protection in an accident. I don't think anybody would make such an assumption. It DOES have it's advantages, though, so don't knock it.