My only experience with pneumatic starters is in heavy trucks. For years, some fleets (notably Consolidated Freightways) have used pneumatic starters on their semis.
Like you said, the motor is lighter, and the truck already has an air compressing system onboard. Also, I think they start a little better in very cold weather. No air pressure? Roll another truck up to yours, connect air hoses and air up your tank.
Real savings in batteries too, as they now run only the lights and radios, and a smaller cheaper alternator also.
They sound really cool too, a vvvvvVVVVing through the fuel tanks. But that's another story.