We've written thousands of words on this topic already over the years, and I'll try to limit it to only dozens more here. Despite the thousands of words on the topic, I view the DVR landscape very simplistically. These three statements are NOT mutual exclusive:
1. DVRs have significantly decreased commercial viewing
2. DVRs add significantly to the commercial ratings (particularly vs. the live numbers)
3. So far, the Live+7 program ratings offer no predictive benefit in determining which broadcast shows will be renewed/cancelled vs. the Live+SD numbers received weeks earlier.
I don't have any difficulty wrapping my brain around all three of the above being true (because they are!). I know some people have a problem with the first two both being true. They are both true. If there were no DVRs there would be more commercial viewing (#1). But there are DVRs, so some commercials get viewed on them (#2) and it adds up to more than just the live viewing by a significant percentage. At the end of the day, for our purposes, I'm far more interested in the third item above.
The Live+7 program ratings haven't added any predictive value in determining how a show is doing. Whether that's true, at least in part, because some of the DVR viewing is already baked into the Live+SD numbers is inconsequential for purposes of predicting show renewals and cancellations.
It's always harder to figure out what's going on at the cable networks, but here's a general rule of thumb that works well: shows that have very low relative Live+SD ratings don't get saved because of their Live+7 ratings. Stargate Universe, Caprica, Terriers, Rubicon, etc, are all canceled. The DVR numbers didn't save them. That of course is also true in the broadcast world (e.g. Lonestar, Undercovers and The Good Guys).