Nielsen is confusing device context with user intent. All that we can really know about mobile users is that they're on a small screen, and we can't divine user intent from that. Just because I'm on a small screen doesn't mean I'm interested in less content or want to do less.
Stripping out content from a mobile website is like a book author stripping out chapters from a paperback just because it's smaller. We use our phones for everything now; there's no such thing as "this is mobile content, and this is not."Of course there's some truth to that. Mobile web usage is skyrocketing, and it's increasingly difficult to say with confidence that you know much of anything about the mobile user. When mobile web browsers sucked, you could say that you were designing a mobile site just to alleviate the suffering of trying to view the regular website on that tiny screen. Now, though, iPad, iPhone, and Android users might be perfectly happy with the same view as desktop users.
However, there's a subtle risk to Clark's point of view, because it gives you a reason not to make a choice. Nielsen's approach is to choose a user scenario and optimize for that; Clark appears to be arguing that you should optimize for everyone, in every scenario. And, if that seems like a difficult task, his advice is to just do it better:
Responsive design, adaptive design, progressive enhancement, and progressive disclosure give us the technical tools we need to create a single website that works well on all sites. We're still learning to use those tools the right way. Just because it's a design challenge to use them correctly doesn't mean we shouldn't strive to do it right.This is great advice for creative geniuses. For everyone else, it's a recipe for failure. I was once peripherally involved in a website redesign project in which the target audience was carefully mapped and defined and put into a series of boxes that, taken together, amounted to everyone who can read. Even people who almost certainly lack the physical capacity to access the website were included in the target audience. It was a complete planning disaster, because when you design for everyone, you design for no one.
That project eventually went off the rails, and it was no surprise to anyone, because one thing that had clearly been established is that the people in charge of the project were not making decisions. Design is about decisions. It's about cutting the cake into smaller pieces, and discarding what you're not going to use. It can be agonizing; the second-guessing can go on forever. But if you choose instead to make no decisions and build your site for everyone, you are headed down the path of creating a site that will delight no one.
Clark, in short, is both right and wrong. He's completely right that we know nothing about mobile users other than that they are using a mobile device, but he is completely wrong in arguing that it is a mistake to optimize for a mobile scenario. Even if the scenario you choose is only partially correct for your visitors, you still stand a better chance of creating a site that users will want to visit.