Maybe that trope served a purpose at one point in time, but in this economy, a lot of people are unable to move out of their parents' place or have had to move back in with them simply because they can't afford a place of their own, even when they have a job. Yet living with your parents as an adult still continues to be used as a primary trait of overgrown manchildren in media, and all it does is make those of us who have had to move back in with our parents feel worse about our situations.
Not to mention, it doesn't even hold up outside of America. In a lot of other countries, especially Spanish-speaking countries, it's common for multiple generations to live in the same household even when all family members are adults.