WMV is proprietary, yes, but it still shares the same basic low level functionality (eg. iDCT, inverse quantizer, motion compensated prediction, etc) as pretty much all the other codecs, so can still be hardware accelerated in part. 
Also, because it's the low level stuff thats done in hardware (rather than having dedicated support for each codec), you can even use part of the decoding hardware to speed up 3D applications - if you know how to access it.