Many are not worth the extra cost. This includes some that were filmed in 2D and are being converted in post production. The problem is that 3D does not add to many movies and are usually limited to action, sci-fi, and animated films. You won't see any drama or love films in 3D. Already you see the theaters slowing down converting a house to handle 3D which includes a new projector and a matching screen. So soon, I think you might see them dropping the added fee so they don't have to dedicate 2 houses for a 2D and a 3D version of the same flick.
BTW, the next big 3D conversion is going to be both Star Wars trilogies, released one episode per year.