Good answers here.

It depends on the bike. A valuable bike that looks experienced should be left alone, since it is original only once and the patina tells a story of its history. A rider that is not inherently valuable could be painted with nothing lost. If the paint is trashed then obviously you can paint it and probably should.

If you just want it shiny and looking new, then it's a tougher call. Lots of folks write their first post to BF in a frenzy of excitement about "restoring to original condition" a bike they just picked up. That type of enthusiasm is great but can cloud your judgment. Sometimes the bike just isn't worth the cost of the paint job. Or the bike will be worth less on re-sale, but the excited owner thinks a shiny bike must be worth more. So you have to consider the bike.

I have bikes I would not re-paint. I did repaint two bikes, one valuable, but a previous owner had already spray-painted over them.