At the risk of starting a debate ... simply painting an old Mac is not necessarily destroying it, especially if the case is badly yellowed, scratched, cracked or otherwise in poor shape, as many, many of them are thanks to poor storage conditions since they fell into disuse. It's not really any different than painting an old black Mustang in need of a new paint-job orange when restoring it.
That said, painting for taste is certainly not destructive. It's not for everyone, but if having an orange Compact makes something more useful or fun to you, then go for it. Back in the day, cases were routinely modified with things like security bolts, chemical adhesives, like epoxy and used on property tags and serial numbers, mouse holders, anti-glare screens, etc. At least painting them can make them look nice.
Since the iMac, most people today expect a choice of color in almost anything they buy, including electronics. Even today, the idea of painting the iPhone is not only desired, but commands a premium for the option.
http://www.colorwarepc.com/p-103-iphone.aspx
In the end, I would have to argue that the only destructive aspect of painting an old Mac is if it could otherwise be restored to factory condition, a very rare find in any event (in which case sell it and use the profit to buy one in poor cosmetic shape), or if the paint job is of poor quality. Certainly one would not want to just buy any old can of spray-paint and have at any plastic case.