There's as many reasons for being into old bikes as there are people into old bikes.

Bikes are interesting; unless they're really special bikes in really special condition- they're generally not worth the sum of their parts. A good repaint is more often than not more costly than the bike itself (notice I said good repaint).

I would not be interested in a bike if it were repainted- to me, a repaint means it was either beat, abused or something was wrong with it. A bike would have to be really special to me and in really poor aesthetic condition for me to consider a repaint on one of my bikes. I'm not so concerned about parts being swapped out- but the paint and decals are more of what the bike is. There was a guy here that was outrageously proud of the year and model of his bike- started a million and half threads proclaiming the year and model of his bike- and then proceeded to braze **** onto it, and repaint it and turn it into something that it wasn't.

As far as parts go- I used to be very much in the 'leave it stock' camp, but as I've experienced more bike stuff, the technology of bike stuff has improved leaps and bounds even over the past 30 years. I think it's kind of silly to have a rider that you're intentionally keeping an inferior ride. Again, it all depends on what the bike is, and what it is to you.