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?