There are multiple different levels of functionality present within 'purevideo' depending on the chipset used. Older chipsets offloaded mpeg1/2 decoding and a subset of mpeg4, later ones will handle the entire stream.
What you've got to ask yourself is what would be the point financially for nvidia to support an obsolete product with new software which wouldn't meet the requirements of anyone who owns it (ie stutter free decoding of h.264 1080i50). It would require considerable extra coding, validation and verification to absolutely no benefit to them or the customer base. A bit irritating if you have an older card that you bought assuming it would be supported at some point, but this is always the risk you run with technology purchases
The best concise description of the features of the various chipsets wrt purevideo I could find is on wikipedia