One reason I don't like them is I don't like the way they look.  They are really only good for skiing exposure, where if you fall you could hit rocks or if you're sledding.  When you're skiing resort, the snow is generally hard, so if you fall with a full face, and the chin guard hits the snow, it can catch and can torque your neck in unholy ways.  Unless you're skiing things like ian mcintosh or eric hjorliefson ski, in the same types of conditions, it can be both stupid looking and unsafe.  I don't know about you, but I'd rather chip some teeth or get a black eye then snap my neck.