Not sure I understand the need for this one as a user could just select the specific (HD or SD) channel for that subscription...Unless I'm missing something...
Some people still prefer to record in SD for certain programs, as it saves disk space and they don't need HD for some program types, like old 4:3 sitcoms.
In Australia we have several broadcasters that broadcast basically the same content in SD and HD in two separate channels, but it isn't that simple, as the programs don't always match. There are always exceptions. Anyway, it is pretty common for these broadcasters to show, for example, the pilot of a new series in HD, and then switch to SD for the following episodes. Or they may just switch part way through a season for no apparent reason, and without announcing it. While I could, for example, just specify the two channels involved in the Recording Rules, I don't actually want to record both, and I do want to be able to specify my format preference. After all, if the program is being shown on both channels simultaneously, one in SD and one in HD, which one should MC record? But if I prefer HD, and a schedule change means that the HD version isn't broadcast for one particular episode, I want MC to record the SD instead.
A specific example is the MotoGP, which is broadcast on channel 1 in HD for every race, except the Australian race, when it is broadcast on channel 10 in SD. Why they do that is a mystery really, except that there is a law in Australia that says the broadcaster's main channel must be SD, so that "everyone" with a TV can view it. HD channels and the content delivered is up to the broadcaster. As the broadcaster wants the maximum number of viewers for the local MotoGP race, they broadcast on their main channel, which is channel 10 in SD. Lots of people are trying to change that silly law now, as it was written for the changeover from analogue to digital TV, and the early days of digital TV here were all SD.
So now I can select Channels 1 and 10 in the recording rule, select a preference for HD, still only get one recording, and not miss any program when the broadcasters muck around with the schedule.
Our broadcasters pretty much don't care about viewers at all, except as numbers of people glued to the TV. They hate PVRs, and commercial skipping capabilities. They change their schedule around on a whim. Miss episodes in a series and don't catch up later. Broadcast last years season, alongside a season from five years ago, etc. etc. So the more rules I have to weed out the rubbish, the better.