It doesn't have to. I had only 119 for awhile and everything worked fine.
The one thing that only comes off 110 that you would definitely be interested in is the 9 day guide. If you only have 119, your guide will only go out about 48 hours.
It basically comes down to what you want to watch. If you have AT60, pretty much everything is on 119.
Things to think about.
Is there anything on 110 that you want to watch?
If you get locals, where are they?
You do have to have both receivers in the 721 hooked up, even if it is only to 119.