Tell that to the ABC O&O's who shove the Live Well Network in HD on .2 and in SD on .3 so the channel is available to SD only customers since the cable companies refused to provide it to them if it was only available in HD. Word is that it was forced on them by the management despite the protests of the engineers. The CW 100+ group is also trying to get their affiliates to carry them in HD on .2 as well. The bulk of them use variable bitrates, so during rapid action motion scenes like sporting events the result looks like crap. WPVI is a nightmare when they have Big East on 6.1 and ABC's afternoon game on 6.2
An OTA station can't carry more than one HD stream; there just are not enough bits in 19.39 to do that.
Rabbit Ears has a list of all stations currently having at least two HD subchannels. (There's even a station in Maine that has 3 HD subchannels)