That's why a lot of people complained about 3D movies like Clash of the Titans and the 3D parts of Superman Returns. They paid $13-$20 for a ticket and got something that looked like that dinosaur UFO shoebox there. Sure, you can convert 2D movies to 3D better than that, but it costs a lot more time and money, words that make many studio heads cough violently.
Fun Activity: Cut out the characters in this Clash of the Titans screenshot. Congratulations, you just saved yourself $5.
On the other hand, movies like Avatar were actually shot with a 3D camera, and though people may bicker about how shitty the story was, they have nothing but praise for the 3D and the visuals. Even people who thought it was a piece of shit thought it was a beautiful piece of shit.
Like Treasure Planet. Seriously, get that movie and watch it with the sound off.
CG animated movies are a slightly different case. All you need to turn Toy Story into a real 3D movie is to make a second virtual "camera," and since it's not an actual camera, it's just a piece of code, it costs virtually nothing. That's why studio heads don't have a coughing fit about shooting animated movies in 3D, and why every animated blockbuster is coming out in 3D nowadays.
Christmas 2011! Save the date!
So don't give studios extra money for just turning their 2D movie to a diorama. It didn't cost them, why should it cost you?