I'd draw a parallel to storyboarding for animation here [1]. Most storyboarding artists get their start as animators themselves, so they know the "form and the material and process," so to speak. But when they do storyboarding work, they need to work in an agile and rapid fashion, to consider a user's flow through the "application" at such a high level that they would be hindered by trying to draw every single detail. Should they know innately what is and is not feasible to animate? Absolutely! But this does not mean that they should use the same tools and techniques as animators to perform their work.

Responsive may be difficult to capture in static comps, but when these comps can be made much more quickly, attention can be given to user experience overall, not just isolated fragments of user interface. And it's difficult to deny Apple's success at driving conversions and purchases.

[1] http://en.wikipedia.org/wiki/Storyboard