I played with this for a bit and found it too simple. If you don't draw the example shapes exactly, it confuses them. I recommend playing with "delete" versus "x" from the example shapes to see just how poorly this does. I could not get it to consistently differentiate between different drawing techniques.

This would certainly get you started for gesture interfaces, where drawing a shape the same way every time is expected. It would not be a good fit for the use case here of diagramming.