I guess people do what their consience tells them...
Personally I've recently broke a G40, it was a pretty rusty shell. Of course it could have been rebuilt, but not by me - I don't have the time or application. But what makes a G great isn't the shell, it's the running gear. All the mechanicals are going into a nice unrusty Mk2 squareback, which is around 100kgs lighter, and will make it all the more special IMO.
Any car, no matter how far gone is restorable, but lets be realistic. I for one actually live in the real world - at least some of the time!
As someone said before, 'feel free to break it, unless it's the last example on the road'. Mine wasn't, so I'm not bothered.