metacity should fallback to non-composing mode when used with an X server which disabled the composing extension even if composing_manager is on. aborting leaves the user without a window manager and in deep trouble, which is not acceptable metacity-2.15.5-6.1
I think we decided to not ship metacity with compositor enabled, so this may not be so relevant anymore.
Closing bugs related to the metacity compositor, since we no longer develop it.