X-Video Motion Compensation (XvMC) was useful many years ago for video acceleration, but these days it's next to useless for modern video formats and hardware. The Video Decode and API Presentation API for Unix (VDPAU) is a superior video acceleration API and it's now been supported well within Gallium3D for some time. XvMC was just good for motion compensation and iDCT operations on MPEG-2 video.
While VDPAU originated as a video API within NVIDIA's binary driver, the open-source coverage is now good, especially with open-source AMD UVD support since last year -- and is compatible with the VDPAU state tracker -- and there's also been Nouveau video decoding improvements too.
Anyhow, with a Git commit made on Monday night the XvMC support inside Mesa is now disabled by default. Tom Stellard did the change disabling XvMC on the basis that its unit tests are failing for the R300 and R600 Gallium3D drivers.
For those still needing XvMC support by Mesa drivers, the support can still be enabled by setting the --enable-xvmc option at configure time.