I'll admit to a personal bias here but I really don't think it is a part of my thinking when I say that I find it incredible that Sam Cook didn't go to the West Indies and indeed rarely gets a mention in these discussions. If Broad and Anderson are being picked then it's relative non-issue compared to England's batting woes. I don't really understand, though, why he isn't one of the first names mentioned if they're not being picked, especially after the success of Robinson's selection last summer.
Robinson isn't express but bowls accurately and moves the ball just enough, as ImpatientLime says above. It was noted last summer after his debut that despite county cricket's current reputation, picking players with great county records actually does work sometimes after all. The description of Robinson also applies extremely well to Cook, excepting that he is if anything even more economical, slightly quicker and with a better fitness record, albeit an inch or two shorter and a worse batsman.
I didn't and don't agree with the decision to drop Broad and Anderson but if you're going to do that then I struggle to see an argument that Cook isn't in the top three remaining quicks in the country. In fact you could make an argument, at the very least, that he's in the top one (IMO).