Unfortunately, this falls into the category "neat but useless". There is next to no application support for XvMC, and partial offload of MPEG2 decoding isn't particularly exciting.
First, let me just thank you for the outpouring of support you provide for adding support for hardware features. Truly inspirational.
To your specific points -- mplayer, basically the only application that matters when it comes to video playback, supports XvMC just fine. And if you have an old card, chances are your CPU matches the card's age, which means that it might actually be useful to be able to watch ATSC/DVB-T streams.