People seem to eager these days to portray the EU as some enemy to be defeated -- not on the battelfield, of course, but certainly at the negotiating table. It should not be this way.
But this shouldn't really be about that. If our message to the EU is that we will leave come what may, might they not in turn regard this as an excuse to impose punitive measures? It's not like they would have anything to lose, or at least not nearly as much as we would. So you could end up with a rotten deal indeed. Better, then, to leave options open.
I stress that I don't expect this to be an option we'll take up -- it would presumably have to be a pretty rotten deal indeed -- but to imply that nothing would stop the UK from leaving is fairly incredible.
Most likely, though, this is a purely hypothetical discussion. If the UK is still a member of the EU by 2020 I'd be amazed.