The only reason DVR viewing should count is because it lets you gauge how many people follow a show. While this isn't worth much to advertisers, it may be worth something to the network that ends up selling the show on DVD.
Which isn't much of a revenue stream compared with Nielsens. That's the problem with downloading/DVDs/DVR ratings - they just don't add up to much revenue yet in comparison with Nielsens. Not enough to make a difference except maybe if you add them all together in a borderline case. Maybe they could save a
Chuck but not a
Dollhouse. (And if
Chuck is saved, it will be in large part because of another revenue stream we're not discussing - in show product placements. Eat at Subway!

)
Also the network isn't always the one who sells the show on DVD. If the studio is not part of the network, then the studio would sell the DVD - who has the rights to the DVD sales varies by show.
DVR ratings might help get a d2DVD movie made
Direct to DVD really isn't a solution. Then you lose the inherent advertising you get from being on broadcast and you have to spend more money marketing the DVD. If the audience wasn't big enough to support the show on broadcast, why would it be big enough to support production costs of the show plus increased marketing costs?
The only direct to DVD case I can think of for a cancelled show is
Futurama.
Stargate's direct to DVD sales are still part of a successful franchise. It's never going to be an option for most shows, because the fundamental problem remains - they appeal to too few people to pay for their production.
Firefly has managed to be a moneymaker for Fox, even though it got canceled and the movie didn't do that well.
But direct to DVD can't support the production of
Firefly on an ongoing basis, or else that would be happening now.