Sorry to bring up an old thread, but I was wondering the same thing, and assuming you don't foresee yourself ever using a DVI monitor or an analog converter (like ever), wouldn't it not matter anyways? Furthermore, since TV shows are in the cloud already, the SD copy could always be redownloaded at any time if needed.
Edit: I also noticed that on my season pass of Walking Dead 2, when a new show is available and I click "Check available downloads" from the Store menu in iTunes, it only downloads the HD version and not the SD version. I have to actually navigate to the SD version of the TV show in the iTunes store and it shows me an available "download" button, so it seems like Apple is only pushing for HD only in it's TV shows. I also really don't understand how deleting the SD copy affects the DRM and playback at all.