I ride in traffic, and because of the laws and enforcement that's usually at 2/3 to 1/2 or less the speed of the cars and other large vehicles...so if it wasn't so incredibly hot in the summer, I'd ride with my FF MC helmet all the time. So during the hottest months, I'm wearing a kind of half-helmet most often used for rollerblading.
I've never needed the capabilities of the FF as there's never been a crash on the road, but someday it will probably happen, when a car finally cuts me off in a way I can't predict or avoid with even the best of defensive driving, and when it does at least my head will be protected (even if it breaks off and rolls down the road by itself :lol

.
I've had one incident where I was essentially run off the road and rolled and skidded on gravel and into a utility pole, and even with my half-helmet at the time I still got a light concussion.
Another incident where I was so exhausted from work and stress that I think I may have dozed off riding thru a parking lot along my path, and rode into bushes (but some signs pointed to possibly being bumped by a car; since I dont' remember anything directly about it I can't be sure what happened. Except what I typed up in my posts about it, I have no info at all on it). The half-helmet helped, but some scratches on me would have been prevented by the FF, and probably my glasses wouldn't have been broken.
Both of those were at much lower speeds, probably 10MPH or less by the point of impact. At 20MPH and up I would most definitely want a FF (and probably leathers/etc).
Regarding availability, I did go out to buy one just for the purpose (though I actually bought it becuase of the Death Race

), but I got it used from Goodwill for really cheap, after trying each one I found for a while to find one that fit well and was in good condition. I think it was $12, IIRC, for half-price sale from $25, but I might be misremembering that. Been a while.