You will get as many opinions are there are car owners on this subject.
I would agree that its a good idea to keep the rare cars original. But I get tired of hearing about how the "rare" cars are important pieces of history, and /6 cars only exist so we can cut them up and make hot rods. To me the /6 cars are just as much a part of history as the 440/hemi cars.
But with that said, look what I doing with my /6 68 dart. I just pulled out the /6 and threw it away in favor of a slightly modified 318 that I had sitting freshly built in the garage.
I also swapped in an 8-3/4 rear, sbp disk brakes with a booster, a front sway bar, CDplayer, and made some mods under the hood to "clean it up" and make it nicer to look at. (basically welding up unused holes, re-routed wires, etc).
I will continue to make changes that make the car handle better, run better, make it more comfortable to ride in, (like better, modern bucket seats). Maybe even someday i'll have fuel injection on it and a gearvendors overdrive.
But I will keep the car "looking" as original as possible on the outside. I like the looks of the old cars, but need some of the comforts of a modern car, because my car is being built to drive, not just to look pretty.
I am currently cutting up my 71 Scamp, thats where i got the 318, 8-3/4", disk brakes, etc, etc, for the Dart.
But even though the scamp was very rough bodywise and rusted badly in the quarters, it still bothers me very much that I'm cutting it up and that it will no longer exist as a car on this planet. A 71 Scamp is not that rare, but every time I pull off a part to sell, or cut off a piece to throw away, a little piece of me dies with it each time.
So over all, I guess my opinion is that these cars should be restored, restified, repaired, or hotrodded, but they should be driven, not kept in storage by snooty collectors as investments, or "pieces of history".