To me they look daft, neither a XC lid or a FF helmet, sort of an inbetween that does not look like it will do the job of a FF – which is what the face protector bit is for, and what the user bought it for.
They look too fragile, they look like the force of a crash will rotate your head or snap the flimsy chin protector. Compare the helmet with a FF, there is more helmet, more padding and more gripping your head with a FF.
I imagine they give you a mental feeling of protection rather than actual protection you would get from a proper FF helmet.
If your going to ride rocky DH, sure, use a proper FF helmet. Otherwise just use a XC lid.