I will agree that some conversions I've seen are botched pretty badly, but I feel the braze ons can be removed properly (with a torch, not with a hacksaw) if they do not suit the purpose of the owner.
If, some day down the road, someone obtains a frame that has had some braze ons removed, guess what? They can be re-brazed, if desired. Paint? Why not if it is deemed beneficial or desireable by the owner?

I really don't see the big deal here - I see some "attachment" issues emerging in this thread.

My frankenstein? Yep. It's a beautiful '83 Marinoni that I sent to the factory in Montreal to have the derailleur hanger and left shifter boss removed (plus the poorly brazed FD hanger installed by a previous owner) then re-painted. When I inquired, I was told that the work was done by Guiseppe himself, and that folks in the workshop were eager to see my conversion (I had given an outline of my planned build).
My build is almost complete, and has been by design from day one - IGH in back (SA S3X currently) with shifter on the right boss, Honjo fenders, Brooks Swift Saddle, Paul Racer Medium brakes, and a front porteur rack. I just built up the S3X wheel last night (Ambrosio Excellight SSC rim).
The bike is stunning, and will be my town bike and sporty runabout from this point forward. I may put an 11 speed (IGH) out back one day.
After 35 years of committed (i.e. other than childhood) riding, I'm done with derailluers - I'm at a stage in my life (middle age and happy to be here) where I can do everything I need with an IGH bike. Why would I choose to put $2000 + into a conversion and leave un-needed bits hanging off of the frame?
If, after I die, someone finds the frame useful and does not like my conversion, they are free to re-braze and search for components that will have been out of production for decades.