I also agree with Nielsen when he writes:
“When people access sites using mobile devices, their measured usability is much higher for mobile sites than for full sites.”
But from this he draws the wrong conclusion, that we should continue making special mobile websites. I believe that special mobile websites is like sticking plaster over the problem; we generally shouldn’t have separate mobile websites, anymore than we should have separate screen reader websites. The reason many “full websites” are unusable on mobile phones is because many full websites are unusable on any device.
Precisely. Mobile should be seen as an opportunity to do some house-cleaning and fix your site for everyone, regardless of what device they’re on. A responsive website is the perfect excuse to go through your current online presence and strip away non-essential features and cruft, and to better categorize and organize your content.
Building and maintaining two separate sites is paradoxical in a way: perversely it’s sometimes cheaper and faster to build two sites than to build one, awesome and responsive site that serves the same content to everyone. I think this is mostly because of the fact that responsive webdesign is still an expert topic and no good design tools exist to facilitate this process for non-coding designers (or non-designing developers), and because of the fact that earlier versions of IE have no media-query support, which I think scares off a lot of developers/project managers from attempting it*.
* I (and several others) have come up with a solution to that problem though, if that’s what was holding you back.