Because being 7ft tall and on top of a massive unicycle isn’t visible enough! 
I seem to be visible enough to have at least 5 people per ride whip out their phone (while driving) to film me. I usually shout to them to put their phone away while they’re driving, that’s dangerous!
I wished I had a full face helmet on today.
Riding the footpath (aka sidewalk). Coming up a moderate hill, then across a heavily cambered road with a considerable grade to the right, down a dip at the kerb then hard left into a steep climb coming out of it. Reasonably technical for road but nothing like muni. I have done it dozens of times before and hit it pretty reliably most of the time. Probably taking it a little bit faster than usual as I am with most challenges now because I am a lot stronger than I used to be.
Can’t be sure what went wrong but coming out of the gutter I UPDd. Maybe I didn’t quite have control from the side slope as I hit the gutter. Maybe I pushed a moment too soon. Maybe the cranks were just in the wrong place.
In a blink I was heading for the concrete. No time to do anything to avoid it and I expected a six point landing, knees elbows and wrist braces. Unfortunately it became seven point. Fat lip and a chipped tooth. Usually I would have my arms out a bit more but this time they folded under me and being an incline, the concrete was closer than usual. Just bumped but a little more and I could have easily knocked my teeth out.
The unexpected can happen and one doesn’t always have the opportunity to take sufficient evasive action. I would never have imagined a full face helmet would be useful where I ride. People judge their risks and make their choices. Today I lost my gamble. Will I start wearing a full face helmet? No. Will I get a mouth guard? Maybe I should consider it.
That’s absolutely the UPD that I always fear 
I always sort of imagined that a peaked MTB helmet would be enough to avoid smashing your face on the road, but I guess if you down chin-first that’s not gonna help much.
Sorry to hear about your crash; I’ve certainly had similar experiences.
Dave Goldberg died of head injuries sustained falling off a treadmill. Freak accidents happen. That doesn’t mean you’ll see me wearing a helmet the next time I’m using a treadmill. (Unless it’s on a unicycle, which would seem like a helmet-worthy activity to me).
Riding on roads, I’m pretty sure that a full-face helmet’s additional risk from restricted sound and vision would more than counterbalance the protection it gives in plausible accident scenarios.
Commiserations on your accident and I hope you get over it quickly.
I don’t think that level of damage would be enough to even make me think about wearing a full face helmet. I spent a couple of hours having stitches put in my face (under local - I really, really don’t recommend that) and a couple of nights in hospital after crashing my mountain bike - that was sufficient to make me think long and hard about a full face, but I still don’t wear one for similar riding as it was a pretty freak occurrence.
As for what I wear when riding a uni - I use one a lot for local transport and wear no special kit at all (apart from shoes - I put on my skate shoes, when I might otherwise wear something different). No, not even a helmet as my risk assessment is that I won’t come off and hit my head. For muni I wear a helmet and KH gloves - and more often now kneepads, as whilst I don’t expect to need them on most of the stuff I ride, as mentioned above they help with my confidence levels. On the guni on road where I’m riding faster than I can run out just a helmet and the gloves.
I pretty much agree with you on all points - its all about risk assessment. The more important questions for me are, how can I minimise getting into bad/injuring situations in the first place? Mostly through common sense. Don’t ride up the left of cars, make sure drivers notice me, try to avoid riding next to HGV’s, ride predictably and withing my comfort zone when in traffic. A crappy little bit of foam strapped to my head isn’t going to help me if some idiot decides to pull out without looking, so my best bet is to avoid it happening in the first place. I think putting a full-face lid on for the road is counter to this, as it’d drastically decrease all-round visibility/hearing, as well as probably make drivers be less careful around you (though maybe it’d be positive in the way of making people notice you more - hey check that guy with that massive bright helmet on!). Obviously a regular helmet doesn’t have the visibility/hearing problem, but it’s also not going to do a whole lot to save your life either (Sorry, no, those photos of a helmet cracked in half with IT SAVED MY LIFE!!! printed over it don’t prove anything - read up on the safety tests bike helmets go through and how easy it really is to break one…).
So noope, the incident in this thread, while horrible, wouldn’t make me consider a full-face, not for road anyway. It’d just make me try and avoid it happening again in future.
Hey, OneTrack, that really sucks! Sorry to hear about your crash. I know how it feels, I had exactly the same impact just riding a bicycle down the nature strip and dropping off a normal kerb. Totally unexpected, after a lifetime of mtb on firetrail and singletrack, racing and orienteering included. You never know when the god of cycling is going to slip you one just to keep you humble! Hope it makes you stronger!