If its my own bike with a trusted derailleur, I may toss the disc, but rarely would I remove it on a customers bike, its just asking for trouble. I also wouldn't toss it if the disc complemented the rest of the bike in anyway. I tend to leave the Huret, Suntour, or any smaller, less noticeable discs in place. I would toss those huge ugly chrome steel versions that were so common years ago on lower end bikes.