Perfectionism, by itself, is not inherently a bad thing, you're right. What makes it a bad thing is when you put the perfection over something else, such as finishing a project late because it wasn't "perfect", or even to some extremes where some much more neurotic perfectionists I've met have had overly grandiose ideas about themselves, and thought that every idea they had was correct and perfection and that people should be beholden to them and their ideas, because anything else is "wrong and imperfect".
Moral of the story, if you know someone who claims to be, or acts like, a perfectionist, don't scold them for it, unless they're taking it too far. At which point maybe a slight slap on the wrist may be required to get them to realize they're being really nasty, not only to themselves but to others in some cases.