I've got a friend who does this stupid trick in his manual, and yes it does save SOME fuel, BUT you are sacrificing a degree of control over the car. What if you suddenly need to speed up to avoid someone whose about to rear end you, or dive into the next lane to avoid a someone literally trying to 'merge' with you? In both situations, if you can't plant it and get yourself out of trouble, your a sitting duck. In my opinion, diminished ability to accelerate (or NO ability in this case) is equally as bad as diminished braking capability.
Oh and whatever you do, don't turn the engine off altogether while rolling, like a mate's mate did - the steering wheel locked up and he was killed in a head-on (no 5h1t).
Go buy a econobox if you can't afford the petrol.