As a practical measure, the gun still measuring within its proof limits assures a buyer that it is safe to use with the loads proofed for.
Of coures, you still need measuring tools to see if the barrels are within prof limits.
I once had a Grant sidelock crack at the radius. I have a nicely cracked Cogswell & Harrison boxlock action here as well.
Weakening the metal through amateur attempts at re-colour hardening has always been one reason I dislike it.
We can all look at a gun and opine on if it is or isn't likely to be safe.
Proof tests our opinions and tells us to what tolerances it actually is.
Prof is not a legal requirement in the US. That does not mean it does not matter to Americans. A lot of my American clients are very aware of it and careful to insist guns they buy are in proof. It certainly affects price.
That said, I reluctantly re-proof guns. Proof subjects them to severe stress. Don't do it unless you have to.
I'm using Birmingham for re-proofing at the moment. London are bulging barrels at the muzzles again. Of course, they always deny anything is wrong when a spate of guns start to fail out of all reasonable expectation.
