Here are the reasons I dive a ffm
a. No fogging
b. No leaking
c. no jaw fatigue
d. no dry mouth
e. COMS - and this works with teaching classes if the instructor has an additional receiver because he can audibly tell the student what to do
f. comfort in cold water
So to say diving a ffm is "crazy and foolish ad clownish" is a completely close-minded statement and tells me you've obviously never experienced diving one. Maybe you should try it and see for yourself. And maybe reserve your comments about condemning those divers that do choose to dive ffm.
I dive a Guardian FFM as a volunteer diver at an aquarium, where I do a comms talk to the guests from the shark exhibit. I got trained on the proper use of the FFM at the aquarium, and I've done dozens of COMMs dive with it. In my experience, all of the reasons cited above in favor of the FFM are accurate (though I can't speak to comfort in cold water). You just can't do a COMMs dive with a regular mask.
However, there are many reasons I dislike the Guardian FFM:
a. Somewhat counterintuitively, the full face mask has a narrower field of view than a regular dive mask. It really feels like tunnel vision.
b. It is a pain in the butt to equalize with an FFM. You have to either use a nose plug attachment, which is very uncomfortable, or else rely entirely on swallowing and wiggling your jaw.
c. The FFM has five straps that need to be adjusted, and it can take several minutes to get the fit just right.
d. The Guardian FFM has a cheap acrylic face plate, which gets easily scratched, and is particularly awful to look through at the surface.
e. You can't use prescription lenses. That's a big negative for me, and reason alone not to use a FFM.
f. The FFM has valves that have to be turned the right way to prevent water from entering the mask. Not rocket science, but something that's easy to forget, and an additional hassle.
Having dived both regular masks and FFM, I would definitely not use FFM for anything other than a COMMS dive.