While it's a good start, I agree it's far from "We fixed mockups" To your point: The inability to use pre-drawn UI items here is a deal breaker.
It remains flexible enough to play with visual concepts outside the restrictions of a UI kit. That's a good thing in my book.
I've actually been looking for a collaborative sketching tool for quite some time; but using terms like 'live whiteboard.' Seems closer to that.
While it's a good start, I agree it's far from "We fixed mockups" To your point: The inability to use pre-drawn UI items here is a deal breaker.