After reading the 6+ pages of comments in the Phoronix Forums in response to his blog post (and some of the comments being from Mesa developers), he took to the Mesa mailing list.
In this long email, Martin describes some history about the problems he's had with Mesa and KWin. His first issue arose during the KDE SC 4.5 development cycle and the Mesa 7.8 drivers. That time around various features were broken on open-source drivers. Then he moves on to this most recent problem of the OpenGL renderer string changing. Plus lots of other information.
In the end though, "Let's work together on a better composited experience on X11."
The first response was from Zack Rusin. Zack's view was simply that the email Martin wrote is way too long to read and doubting anyone will read it as a result. Zack also argues that KWin, Compiz, and Mutter aren't Mesa's most important downstream uses. He acknowledges that supporting these common window manager are important, but not the most important.
Lucas Stach and Jerome Glisse also responded. Jerome went as far as saying "Maybe here a more radical solution would prove to be better. Just disable compositing as obviously none of the open source driver works properly with your compositor. In couple year maybe the opensource driver will be good enough."