Having recently moved here from Windows Media Center, in progress commercial skipping is the one feature I miss the most. Being able to join a recording in progress of sports, news etc. is incredibly common in my household and automatically skipping the commercials while the recording in progress would be a huge plus.
I agree that commercial skipping a live channel is useful, but it would also be relatively easy for someone to start recording an in progress show (moving the buffer to the DVR) if they wanted to enable commercial skipping. Yes, they need to delete the recording at the end (or wait for it to be deleted automatically), but is that really that hard?