I don't understand why sub channels are desirable?   One has to rationalize what they wish for.   I'd rather see the entire 19.39Mbps of the ATSC stream dedicated almost entirely to the single primary HD feed.

The presence of a sub channel does consume bandwidth that would normally be made available to the primary HD stream.  An SD sub would consume 4-6Mbps of the total 19.39.  The corresponding decrease in bandwidth for the primary HD channel would require higher compression rates and thus a corresponding decrease in quality.  It would be no different than what the cable companies do highly compressing multiple streams in a QAM channel.

I'll settle for the single HD feed without sub-channels any day!

cheers,
N