Reminder on some facts
The ATSC digital OTA broadcast system, using 6MHz channels, allows broadcasters to slice-and-dice each channel in a variety of ways between television programming and data.
Post #15 in the OTA FAQ gives a detailed explanation of sub-channels and bandwidth issues.
If you didn't know that, now you do.
As for whether to do it or not, roger1818 reminds that Canadian policy is for major market DTV stations to use their maximum bandwidth for a single channel.
The CRTC has opened the door to co-located
rural DTV transmitters, in which a group of broadcasters may share tower and channel space. Whether that means discrete full bandwidth HD channels, discrete bandwidth-reduced HD channels with SD sub-channels, or one SD channel with SD sub-channels remains to be seen in policy and practice.
Now, speaking purely as a consumer, I applaud that major city DTV broadcasters must use their full bandwidth for their single channel. The visual benefits to the viewer are not arguable. It is not necessary for DTV broadcasters to slice up their bandwidth allotments with info that can easily be found elsewhere.