I like old japanning. I like old hand tools along with sewing machines and my old Stanley planes were japanned. It's a tough finish. I think why manufacturers moved away from it was more in an effort to reduce costs (plus styles change) than it was because more modern paints were superior. If a machine's finish is toast AND it's not historically significant or rare, then by all means go ahead and paint it. Most of Steve's machines are rare and becoming more significant with every passing day so it makes sense to preserve as much of the original material as possible.
People do repaints of featherweights every day. They're one of the few machines the repainters can sell at a profit when they're done. I think though that eventually people will appreciate a machine with a good original finish more than the repainted ones.
Me, I prefer an original finish or one that has been minimally repaired/restored using traditional materials and methods. I don't collect rare or valuable machines and it's still pretty easy to find machines in good original condition. I also don't mind honest wear on a machine. It's ok for a 100 yr old machine to show it's age a bit. It's natural the finish on the bed will take a beating from all the fabric being dragged across it and from needles poking and scratching it. I'm not as fond of pinrash on the arm of the machine even though it's the result of use too. It must be a useful place for pins for so many machines to have it, it's just too bad there wasn't some sort of protection for the machine.
Rodney