Well I subscribe to the MythTV mailing list, and they are raving about VDPAU.
Not everyone has a beefy PC in use for watching TV, where it may be part of a media centre, and needs to be small and quiet. Both small and quiet mean lower airflow and more heat, and thus a lot of people go with far lower spec PCs for connecting to the TV to keep the noise and heat down.
By contrast, the MythTV setup is often split between the frontend (see above) and a backend, which does all the recording and storing of programs. Because this is often in an office or basement, it can be far beefier, but often isn't used for viewing content.
I have a split setup, but my frontend, built before VDPAU came about, is plenty powerful enough without VDPAU.
Just mentioned it as maybe of interest for someone with older kit that may be struggling otherwise. A video card upgrade for 15-20 quid will give you HD playback on a CPU that isn't capable on its own.