If you want to fault this, you would probably do much better questioning the ability of a user to create refined designs on the level of rocket science with just his hands floating in mid air. There's nothing to press against, nothing to provide feedback. That would require very intricate control indeed.
Right, if you had never done any drafting (by which I mean even just one class in autocad, good old-fashioned drafting, or both) then you might reasonably think that this would be a problem. But since you can draw shapes and then re-dimension them afterwards, or you can draw shapes constrained to dimensions, in practice this isn't really a limitation. It's just another issue which has to be taken into account when designing the interface. Perhaps arm motion will handle large-scale motion and finger motion will provide fine adjustment. In any case, just like gorilla arm syndrome, this is an issue which can be "solved" in the interface design.