If you only wanted carrier IPTV service, your set-top box would get IP access to the carrier's local video servers, not the Internet.
If you only wanted carrier voice access, your ATA would get IP access to the carrier's local voice servers, not the Internet.
Its no different than today if you ordered cable TV service, but not cable modem service. Or order telephone service, but not DSL Internet service.
People would be buying access to different "virtual" networks. You could buy access to the video network, the voice network or the Internet network. Just because you buy access to one of the networks, doesn't mean you get access to all of the bandwidth on the physical circuit.
If you don't buy/use the carrier's voice or video service, the Internet service is effectively the only service on the DSL access link, so QOS just acts as a bandwidth limiter based on the access rate you bought. In that case, there is nothing to "prioritize" beyond a few link management messages.
The carrier could offer "burstable" Internet access up to the link rate, but would people understand what happens when they use more bandwidth than exists on their access line when they are sharing bandwidth among all the services instead of reserving fixed amounts of each service?
Tech savvy people may understand they have a total of X-Mbps of bandwidth. When they turn on 10 HD video streams, will they be surprised if they see macroblocking. Other people probably will call their service provider to complain their TV doesn't work or they aren't getting the full X-Mbps downloads at the same time as watching HD Sports on their TV.
In the near term, under-promising so you can over-deliver seems a safer path.