I'd love to own a historic bike like the sandcast 750, but then, I like to ride my machines often. I'd hate to have something so rare and nice that I'd never want to ride it for fear of dumping it... (IMHO)
In 10 years I've put over 20,000 miles on sandcasts. If they're running and weather is good, I ain't afraid to ride 'em, and ride them hard. One was my daily beater for years, and it was friggin fast, too. Pegging them for all they're worth part of the motorcycle game- the early ones will achieve 125mph if you have room and lay on the tank. They're valuable not just because they're rare, they are CB750's in the PUREST sense and when properly tuned, extremely satisfying to ride. Plus the fact whenever you park at any bike hangout, a conversation piece to say the least. And an exhaust bark to back it up.
When they're in super concours condition and up for sale, its a different story. Its not prudent put miles on them. Sort of like a 0 mile collectable, they lose value if used. A garden-variety used sandcast in nice original condition won't lose any value if ridden. Unless you drop it, but that advice is same for ANY motorcycle except dirt bikes.