I could only speak for myself, but coming from a design background, I find that while designing a web page there is a great deal of experimentation as you develop your Photoshop mockup. A texture here, an image there, things that may not make the final comp but experiments that help you create something original. That seems to be lost or at least less intuitive, with this method. Even if I were a fast coder I feel I might lose that free thinking time and end up with a more rigid, less artistic page. Maybe that is where the difference lies between a web developer from a design background and a wed developer from a programming background.