An example of where this could be useful, at least for me, is the small formfactor Compaq Pentium III box I have. If I had some cash to spare, I could pick that up along with a blue ray player and install them then plug the thing into my TV. While it wouldn't be as graceful as a dedicated bluray player, it might be cheaper and it'd be more versatile. Get a capture card to go with it and I've got a poor-man's all in one DVR solution.
Also might be decent for one of the many Opteron and other relatively powerful systems made last decade that were stuck with a motherboard lacking AGP slot due to their original intended role as a server.
The card should also be able to provide hardware acceleration for Flash and other various softwares that now use the GPU despite having nothing 3D (Firefox, etc). Basically, in theory, the card should be able to add sufficient multimedia ability to a decade or less old computer that it can be somewhat useful for more than old games or word processing again. It might also catch on in educational institutions and government offices where you have a ton of aging and obsolete computers but not the resources to replace them.