Chucks description is pretty much what I've heard in the past about multi-casting and the allotted bandwidth. I don't think we need to look any further for confirmation of this than searching this AVS forum as it was a hot topic last year & Chuck is 100% on the money. When this was the topic about a year ago people were saying that HDTV might go away in favor of multicasting. Since we all love to worry about what's going to happen, it was pretty much beaten to death. All of the usual AVS experts were saying it would remain to be seen what would happen; that stations are given a limited amount of bandwidth allocated to them & because of this, they might choose to either multicast instead of showing HDTV or lower the data rate of the HDTV in order to do both. Now of course I don't know exactly how much of the total bandwidth an HDTV show takes, but most people seemed to feel it took nearly the whole bandwidth.
Here is a very generic example. (this is just an example so don't read into my 'unit' example) If a station is given 10 units of bandwidth for their station, they have a few choices. They can chop up the units into separate channels or use all of the bandwidth for an HDTV channel. If they show an HDTV program in it's full data-rate, let's say that it might take up 6 units. An SD program might take up 2 units, so showing several channels at the same time might be difficult without lowering the data of the HDTV show or eliminating it totally. So by lowering data-rate of the HDTV program to like 4 units, they can now show both, but at the expense of the HDTV PQ. The other choice, (which was the one everyone was worrying about last year) is to get rid of the HDTV totally (hello Fox) & just show 4 or 5 SD programs.
If they could multicast & show HDTV at the same time without lowering the quality of the HD show, then it would be a non-issue & would not have been debated so much. So in other words, even though 2.1 is a separate channel from 2.2 & 2.3, they each are a fraction of the total bandwidth. Ya know what they say, "The whole is greater than the sum of the parts". That is exactly the case here. If you split the bandwidth into separate stations, it can't equal the power of just one station showing HDTV!!! :)