Why is there a “2D” disc in every Blu-Ray 3D package? Wikipedia says:


  MPEG4-MVC compresses both left and right eye views with a typical 50% overhead compared to equivalent 2D content, and can provide full 1080p resolution backward compatibility with current 2D Blu-ray Disc players.


So if a Blu-Ray 3D is completely backwards compatible to a regular Blu-Ray, why do all consumer Blu-Ray-3D packages contain a 2D as well as a 3D disc? Is that just marketing (so people will even buy it if they don’t yet have a 3D-capable setup) or is there some technical reason behind this?
Disk space.  The 3d content takes more space so there is generally not enough room for special features.  The 2d special features are only on the 2d disk.  They take up the room that the second eye takes on the 3d disk.  They could make a special features only disk, but this is generally more expensive since it requires another master and another production line.  It's more cost effective and better marketing to just include the 2D copy.