Pure device was enabled to get a really tiny performance boost. We should either fix the code to detect the GPUs that won't support hardware vertex processing or remove the Pure device + HardwareVertex change (only the flags since the DirectX debug logging will blow up again and fill the whole world with warnings if we remove the state manager ). Hopefully VertexProcessingCaps will tell enough info to make the sw vs. hw vertex flag selection possible.
A quick comment - there are probably a lot of laptops still in use with older Intel GPUs, which people run MP on mostly for TV use - we would lock all these users out for (if I understand the comments correctly) only a very small performance gain for other users. Also it's not unusual for potential new users to try out MP on an older system - if it won't run on what is still a reasonably modern laptop (or SFF system with no expansion slots) they'll probably move on to other HTPC software....
(Although I wouldn't run MP on it for real use, I installed Win 8 on an old Core2Duo 2 GHz + 945G chipset laptop recently - the video playback performance is actually not that bad - much better than I remember it on it's original Vista OS, it can even manage 1080i video at sensible CPU usage )