Putting pop stars into movies makes a lot of people rich. Those people tend to be the agents, the lawyers, and the marketing companies but not the studios because they all flop! If SHOWING the pop star cant bring the kids into the theater, how can using just their voice do it?
The rule around Hollywood is that using a star can open a movie (e.g., bring in people the first weekend), but after that its up to the movie itself. The problem with Atlantis wasnt Michael J. Fox it was that Atlantis was a rotten movie. You could have had Aaron Carter, all of the Backstreet Boys, Tom Cruise, and a year of pin-ups from Tiger Beat magazine; it wouldnt have made a difference.
There are actors, there are stars and there are celebrities. While they may look similar, they are very different species. Celebrities are useful for marketing auto insurance, but they arent useful for making movies. Stars can be used in movies when the movie itself is weak and the director can use a star rather than having to work at creating a character. Actors, however, are vital if you want to make a good movie. Movies work best when the audience sees the character, not the person playing (or voicing) the role on the screen. Look at the list of the top grossing movies. Very few had stars in them when they were made. People respond to story, not marketing gimmicks.
The fact that Disney switched to stars like Elton John, Phil Collins and Sting for the music in their movies is because Disney lost confidence. At best, it was a poor creative choice. But mostly it was a marketing decision to use flash instead of talent. Instead of creating interesting stories and strong characters, its much cheaper and faster to toss in a name rather than work at creating something good.
As for the music having Elton John do the music for Road to El Dorado sure didnt help that movie at the box office. Why didnt all those people who flocked to see The Lion King for the music go to that one? Or, for that matter, why didnt they show up for Aida?
Lastly, it sure seemed like Beauty and the Beast, Little Mermaid, and The Lion King sure hit it big at the box office without Brittany Spears.