I don't think they get any extra viewers by putting people like Mariah or J-Lo in the judges seat. The show needs to be about the contestants, not the already wildly successful judges. People like Randy Jackson and even Paula Abdul (when sober) kinda made sense because their careers had already matured & moved on. They originally had 1) a record exec, 2) a musician/producer, and 3) a singer/performer. I think this gave a good balance of backgrounds, even if Paula mailed in most of her responses.
Going for the current superstar judge (J-Lo, Steven Tyler, Mariah) is being used as a platform from which they can launch their own personal projects. It kinda annoys me.