Helmets save lives, so yes, they should be mandatory.
In 2011 4,612 motorcycle riders in the U.S. died in crashes. Alternatives are costly and/or ineffective. Also there is no evidence that motorcycle rider training reduces crash risk. Critics of helmets laws often say that helmets cause neck and spinal injuries during crashes and restrict vision, etc. Bollocks; death rates from head injuries are twice as high among motorcyclists in places without all-rider helmet laws. Motorcycle helmets are about 40% effective in preventing motorcyclist deaths and about 70% effective in preventing brain injuries.