There were times, when there was no official Xorg mga driver, but since parts of the offical respectively unofficial drivers are closed source (especially the mga_hal library), this driver is not suitable for inclusion into Open Source only distributions like Debian, Ubuntu, etc.
Additionally, Matrox' driver license prohibits the inclusion of the driver into distributions thus making it even more difficult for distributors to provide out of the box support for Matrox graphics card.
The solution to this problem was to create a new, open source driver using parts of the official driver and allowing people to pimp this driver using the mga_hal library (that can be downloaded from Matrox' website).
This possibility has been dropped some time ago, so the current Xorg mga driver does _NOT_ support linking to the closed source mga_hal library instead the Xorg guys are doing a great job and try to implement all the features that are currently buried in the mga_hal into the new driver by reverse engineering and/or trial-and-error methods to catch up with the features the official driver has.
In the long run, the official/unofficial mga driver will reach its TTL and the Xorg mga driver will fully replace it, once all the features are implemented there, but that might take some time and until this date, people who need special functionality that's not supported in the Xorg mga driver will find the solution to their problems here.
I hope that answers your question.