To each his own.
I can go flying down a hill on my Orbea at 45mph and not think twice with my little Giro helmet, but the few times I've ridden a motor with an open face helmet it was freeky. Go figure.
Ain't that the truth. No problems at all flying down a hill on my bicycle loaded with blind corners at 50+ mph with nothing more than nylon shorts and shirt and a leather "hairnet" helmet. But hop on the motorcycle to go to the store and it's like I'm gearing up for battle.
Oh, regarding the subject: Harley shops carry tons of cheap and effective sunglasses with interchangeable lenses