What was a good case for a desktop app even two years ago works fine on the web today (e.g., WebGL stuff, canvas-based apps.) Chances are this will be even more true 1-2 years from now (when a product like this would have become quite polished.)
A web app can offer cheap distribution, a more iterative development cycle, split testing, more monetization flexibility, piracy protection, a larger market, immunity from platform trends, and virality (share interactive designs even with stakeholders who haven't downloaded or bought a large creative application).
For me, the first half of that list is just as easy natively. Larger market from day one, platform, virulence - I can see that. But what of robustness? Access to lower level features in order to create something huge, fast and efficient. without being held back by the browser. Not to mention do I hate not being able to work without an internet connection.
Thinking on it more, you can leverage so much of both worlds and make the best thing ever.
