i disagree - he's not exactly been a stellar performer, his stuff is late, over budget, under performing, and his media/political skills are so bad that he doesn't even appeal to the 'send 'em to the workhouse' brigade within the electorate.
getting rid allows the government to blame him for failings/bad policy, with the new face not enough time to make too many enemies before the election. he's no threat to Cameron, no one is going to be upset that he's gone, and he makes room for someone deemed more politically attractive and more competant. if that person happens to be a) a woman, b) quite pretty, and c) not Oxford, then all the better for Dave...