It would seem a case of overkill to me. The only piece of research I've come across which looked at the autopsy reports of cyclists who'd died from head injuries was by Dr. Mayer Hilman (UK) in the CTC's Cycle magazine, a good many years ago.
His analysis showed that about 92% of all riders with fatal head inuries would have died anyway, because of other, but slower acting injuries. So the full face helmet would only be of use in those cases where a normal cycle helmet would not have protected the rider. I speculate (admittedly), but it seems possible that any impact requiring such a helmet would be likely to cause other and possibly fatal damage. I would also be slightly wary of the possiblity of the greater size and weight of the helmet increasing the risk of greater rotational forces on the head in the case of a collision.