Rob, you won't be shocked to hear that I don't really agree with that either.
"How many ads should be on the page" is not the same conversation as "how do we make ads more viewable". In fact, those two conversations can have very different results. You can cram a whole lot of ads into that part of the page that you know is most likely to be viewed. It will be obnoxious, but you can do it.
So, viewability does promote conversations about how to please users, in that those conversations are in the direction of "OMG, how are we going to keep the users engaged with our content, while also sticking all these ads in their faces?"
In other words, pages designed for ad viewability and pages designed for good user experience, are, at best, in a Venn diagram with a tiny overlap. But I think it's more accurate to say that those categories are almost antithetical to each other. Users don't care about ad viewability at all. And advertisers don't really care about the user being happy with the publisher's site. They sort of care, in the sense that they want the user to get to the site for some reason, but figuring out that reason is not the advertiser's problem. What they want is for the user to forget about the publisher's site and click on the ad. That is not what the publisher wants. It can't be, because if that happens too often, then the publisher has nothing to sell.