Those who utter this phrase believe it makes them seem impressive. What employer wouldn't want an employee that does everything perfectly, or who at least strives toward it? On its face, it seems an innocuous phrase and perhaps even one that could catch an ear. But it carries several issues.
First, it's just too common. Employers hear far too often that candidates are “perfectionists,” and it simply does nothing to make a person stand out. More importantly, employers know it's not true. No employer has ever hired a perfect employee, so they know that anyone claiming to be perfect can never live up to it.
Most employers also don't want a so-called perfectionist on their staff. They want someone who isn't afraid to make mistakes. That's how employees learn. They also don't want someone who will get so bogged down in minor, ultimately unimportant details that they neglect their other tasks and sacrifice productivity.
Claiming to be a perfectionist can make one seem new to the job market, and new to interviewing. If you want to come off as a savvy worker willing to learn, you'll have to admit to some flaws.