Seems that the discussion at the summit went off on a bit of a tangent. Playing a video stream is a classic example of a soft real time system: there are deadlines but nothing bad happens if you miss them some times, and as Linus pointed out in most cases the behaviour people want is best effort, but try harder in this case.
With hard real time, you must not miss the deadline ever. An example of a system I have worked on recently: printing sell-by dates on bottles going along a production line. If one bottle is not printed the whole production line has to be halted, the problem fixed and then restarted, which is very expensive. This is hard real time.
You can achieve hard real time behaviour with a priority based scheduler, but it is not easy. Much better to use a deadline scheduler which gives you the guarantees that you want. So, I vote very energetically for inclusion of the deadline scheduler. It may not be useful on the desktop or server, but that is a small part of what Linux does.