I don't....but probably should....
Though I've ridden on a few fairly gnarly trails and have actually worn the full-face helmet, the only time that I've actually needed the protection offered by a full-face helmet was on one of the easiest XC trails in the state.
It was Spring Fling #2 (2001) -- and we were riding the TRW trail out at Hawes. This is a trail that I take absolute beginners on. Anyway, the desert was blooming, and it was quite beautiful. And so, I took my hand off the bars, to point out a "blooming ocotillo". (In the old days of Passion, this was quite a joke, btw.)
In an instant, the front of the bike became unstable, and over the bars I went -- I landed on my face. A quick check of my teeth indicated that they were OK, and I rode on. It wasn't until the end of the ride, upon looking in a mirror, that I realized I was going to have a scar from my injuries. A scar right above my lip! A scar on my face!!!! Right in the middle of my face!!!! I started to cry. Men cannot figure out why a woman cries about an injury two hours after it occurred, but there I was, in the midst of several dozen helpless men, quizzically staring at me, bawling my eyes out. Fortunately DurtGurl was there, and she understood the gravity of the situation.
Things got worse, of course, when I realized that I had to go on business travel the very next day -- to a meeting where I was going to meet several people in my work group in person for the first time. With either (1) a gaping wound smack dab in the middle of my face, or (2) a bandaid right along the top of my upper lip. Mortified, I found the goofiest Mickey Mouse bandage, put it right across my upper lip like a Hitler mustache, and just decided to look like the biggest fool on the planet. (Talk about looking like a dork!)
After that, I wondered just what you ask: is it worth it to wear a full-face on XC trails? Sometimes I think it is a very good idea....