Okay, well, bikes from the late 70's are really more like used bikes than vintage bikes.... [/snark]
...
But if you go back a couple decades farther, you get to some really fabulous paint and graphics, some of which really can't be replicated, and in such cases, I much prefer tatty original paint over boring repaint.
I read your comment above and thought of this Joco frame whose bidding just ended on eBay [
Joco frame, winning (auto)bid $50]. All those pinstripe details (and tatty paint) would be lost in a new paint job (or double the refinish cost to reproduce).
Detailing on every tube and the fork too!