Mesa Gallium 3D is not an API intended for use by applications. It is a mechanism that allows to share functionality among drivers. Yes, it is possible to create state trackers on top of it, but that will be part of Mesa. It has no public API.
Even among Mesa drivers, usage of Gallium 3D is not mandatory. If the driver team thinks it will make their job easier, they can use it. If they think it won't, they won't. Intel 965 driver (the current driver for Intel GPUs) doesn't use Gallium, for example. The new one ("Iris"), currently in the works, will. For a long time, the only driver that used Gallium 3D was only the AMD one.
Even among Mesa drivers, usage of Gallium 3D is not mandatory. If the driver team thinks it will make their job easier, they can use it. If they think it won't, they won't. Intel 965 driver (the current driver for Intel GPUs) doesn't use Gallium, for example. The new one ("Iris"), currently in the works, will. For a long time, the only driver that used Gallium 3D was only the AMD one.