We're getting into the area of "true" and "discreet" debate. I think it's important to note that, while EX material does not have a discreet back channel, a properly mixed EX source will play back channel sounds through the back channel speakers.
If you put your ear to a back channel speaker you may occasionally hear sounds that are not intended for the back channel. However, while watching your favorite films, sitting in your sweet spotted seat, (or even off to the sides, front, or back of the room), you will hear the soundtrack as it was intended to be heard.
The simple fact is that you don't need infinite separation between channels to hear separate and individual channels (if you did, the stereo LP would never have come into existence). In other words, matrixing does work.
I wouldn't be surprised if at least the Dolby Digital folks do a discrete version of Dolby Digital EX for HD-DVD (to match DTS-ES discrete), but I was a betting man , wouldn't surprise me at all to see them try to leapfrog them to 7.1 discrete. But who knows?
As far as DPL IIx and Logic 7 go, which right now are probably the 2 best ways of "expanding" a 5.1 soundtrack to 7.1 channels, most people do prefer 7.1 playback to 5.1 playback. But how the system and speakers are set up matters as well. A well done 5.1 system will beat a mediocre 7.1 setup every time. But by the same token, I believe that a well done 7.1 system will be preferred by most people over a well done 5.1 system too.