Apple chose not to open their GUI because a powerful, easy to use, and consistent graphical user interface are always the most successful. Any platform that expects to make major inroads into the consumer space needs to reflect these attributes. With all Linux's strengths in so many areas, these various GUIs continue to be among some of platforms few yet largest stumbling blocks. Most might think that more is better. The reason is because consistency is king for desktop environments.
Computer users require similar functionality throughout the OS to remain productive.
With an open GUI, then only thing that could come of this would be inconsistency between the varying windowing environments. Think of it like Apple having clones all over again, but allowing the company to retain its strengths in key markets where previously the clones fed off of Apple's marketshare. In recent times as I became more aware of Open Source and all its benefits, it was tough for me to cling to a company who was closed source in nature. One day it hit me like a brick on the forehead. I really think that Apple is on to something big here.