There's a huge separation between who designs the frontend and who implements the frontend. To date, none of the frontend editors are good enough to completely replace the need for a programmer to wire up actions. Even back in the Flash days, you needed an ActionScript programmer to make your webapp do anything meaningful.

Since you need a programmer anyway, you might as well use a freeform design tool (Photoshop) to let the designer design whatever the hell he wants without the constraints that a more specialized app might impose. Then, let the programmer take that design and create whatever infrastructure he thinks is necessary without also constraining him to whatever a specialized app might impose.

I've been doing both design and programming a bit lately and I absolutely love apps like Subform, Webflow and Apple's interface builder. But, once I get past the fast iteration mockup phase, the structure of the app or webapp I'm making changes enough that the tools become unwieldy.

What I would really like are more robust layout APIs for both the browser and native apps. In Subform, you can define relative widths and positions both to other components and to the window itself. It's great for taking some of the tediousness out of design, but from a programming perspective, centering an object horizontally and vertically consistently is still a surprisingly difficult endeavor.