A put-up job?

Can't quite see what David Davis hopes to achieve by his resignation as an MP. If he feels so passionately about the 42 day business and civil liberties generally, he surely could have achieved more by staying where he was - you can do much more damage if you're working inside the system, rather than on the outside. As it is a badly kept secret that he and David Cameron don't get on well together, my cynical side wonders if this is simply a cunning way of getting him out of government and onto the back benches without the embarrassment of having him officially resign from the cabinet.