I love this idea very much - mainly because I had virtually the same idea maybe 5 years ago. I was frustrated with existing prototyping/design systems and getting bogged down in details and pixels instead of thinking about the 'big picture' and the workflow (which IMHO is far more important, especially at the early stage).

I had a manual process whereby I would draw all my prototype UIs in pencil on large sheets of paper, then use an A3 scanner to import them into a slideshow. The cumbersome part was creating hot-clickable areas to jump between screens. But it worked a treat, and enabled very fast turnaround of ideas.

I think workflow (or UX if you like) does not get enough attention in the early design phases, where people seem to like producing pixel-perfect mockups in Photoshop. The problem with this is that it takes you a very long way down a particular path, when you don't even know for sure if that is the correct way to go.

Photoshop mockups are time-consuming and expensive to make compared to rough sketching on paper. This app looks like a great solution in this space.