I'm inclined to agree. Running down the IMA battery instead of using the ICE will not help mileage much, since it doesn't store a whole lot of energy to begin with. It's more to give it more power "on demand" so they can get away with using a smaller ICE, and also for regen, of course.
Also, running down the IMA battery instead of using the ICE would be deep cycling the battery, which is not particularly good for it.
You probrably could do this, but I think it would cost more in electricity, (costing money) since regen is essentially free. (Unless you have the distance and lack of traffic to coast to a complete stop, braking will be required, so you might as well capture that energy)