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[Deron Johnson]

The GL solution is a heavyweight solution which, from a systems
engineering perspective has the highest total cost of implementation.

+ It only addresses GL; it doesn't address other types of graphics
  and imaging libraries that a compositing manager may wish to use.

+ It requires two changes to the GL API and associated driver
  interface. Furthermore, The API will need to evolve over the
  next couple of years as it moves from being an EXT extension
  to an ARB extension to part of the GL core. Clients of this
  interface will need to continually adapt to the syntax changes
  which arise from this evolution. And the earlier incarnations
  will need to still be supported for some time.

+ It requires the cooperation of and makes extra work
  for lots of other people in multiple companies: the GL ARB,
  managers, architects, driver developers, test engineers,
  tech writers, etc.

+ It will take orders of magnitude more time to implement than the
  alternative.

+ It doesn't have a definite completion milestone. It will take an
  indefinite amount of time for drivers to be upgraded and even
  then there will always be older devices that won't support it.

On the flip side, the composite priority window is a very lightweight
solution.

+ It addresses these problems for all conceivable graphics and imaging
  libraries.

+ The interface is very simple.

+ The implementation is near to trivial.

+ It has no adverse impact on window managers.

+ It is a simple, isolated solution which doesn't require a cast of
  thousands to make it happen.

In short, the GL solution is like using a pile driver to drive in a nail.