Ha, you have to ask
What goes up must come down...all the crud thrown up by the rear wheels in front. When it gets a bit, well, mud storm'ish, tipping your face forward a bit means the visor catches most of what you face would
MTB's are by definition a lot slower than road bikes but the knobby tyres chuck up great clods of undersirable stuff when the going is a bit sloppy.
No problem if you are out in front mind
Riding in a pack of road bikes without guards in the rain is not really the same as the water is chucked up in a stream - peak or no peak you wear it but I see some road helmets now with full face visors. Not my cup of tea but may work for some.