How does it even have a census of your teeth?
Some people have wisdom teeth, while other people don't.
Some people have been on the losing end of a bar fight with carneys, and some people haven't.
(I imagine a system like this would need at least two accelerometers - one close to the tip, and another one in the grip area; the second one would be used to cancel out the movement of users' head and body, allowing the first one to effectively operate in a mouth-relative coordinate system.)
Maybe not from brushing one tooth but brushing the whole mouth you'd get a pretty good picture I'd say. The app would justify itself if it did give you a rough map of how you brushed your teeth and suggest areas that you're missing/if it's being done incorrectly.