Why do Mountain Bike helmets have visors? Visors seem pretty ubiquitous on mountain bike helmets, but not on road bike helmets.  Why do mountain bike helmets have visors? What purpose does the visor have?
One purpose of a visor is to shield the sun like a baseball cap.  

There are many factors: 


Wind resistance is not as big a factor on a mountain bike (slower speeds).  
The more upright position on a mountain bike puts a visor more into
play. 
On a road bike the rider is leaning forward and facing down and the visor can even block forward view.  
A visor can help keep rain, sprayed up mud, or low branches off the riders face.
Mountain bikers think it's cool?