The studios are likely putting out separate discs for multiple reasons. It could be the space issue or perhaps older players would need some firmware update to be compatible playing back 3D discs in 2D mode. I think the studios also want to differentiate (and thus charge more) for 3D vs. 2D.
Another interesting thing you might notice is that 2D playback has been disabled on the 3D A Christmas Carol BD. The 2010 World Cup BD is apparently mastered this way as well.
