As Agilis mentions, there is a real probability that at some point, this new full face snorkel will get kicked by another diver and flood, or have some other tragic event cause a large leak--or perhaps the one way valve in the snorkel will fail---pretty much any decent scuba diver, and 100% of all free divers, could deal with these issues without more than annoyance.....BUT...put someone prone to near phobic panic with normal snorkel problems that prevent their comfortable snorkeling---and you have this phobic and unreasoning person panicking with the full face mask snorkel system, and potentially drowning if an instructor is not right next to them to SAVE them....
Again, the people that would be put in these masks, would need handholding by an instructor--they might as well go with snuba -- or the raft with the viewing port for the cost of a pony ride.