However, we always (always always) treat the PSD step not as Final Design, but as a high-fidelity communication tool - the next step after our wireframes (which are as un-designed as possible) and story cards. When requirements change, we loop back as far as necessary; for small changes, new stories and in-view UI adjustments suffice; for more fundamental changes we'll go back to the PSD.
Don't get me wrong - being able to maintain design quality in the face of change is one of the most difficult aspects of being a designer on an agile team (and getting design involved in the entire delivery process is absolutely essential) but the tool you use to build initial design isn't necessarily going to solve that challenge for you.
Personally I find it healthy to be able to concept layout & UI without making stylesheet & DOM structure assumptions. Designing via code, for me, blurs my focus - when I'm making decisions in Photoshop, I'm aware of the CSS that will be required to replicate it, but I'm not having to actively execute on them. Designing and coding take very different parts of the brain for me, if that makes any sense. Try drawing a picture while listening to a podcast vs. reading a book while listening to a podcast.
> “When it comes to products, user experience trumps pretty pictures. It’s not easy for most designers to do, because you have to rebel against everything you’ve been taught as a designer: fight the pretty pictures, fight the urge to tweak your drop shadows or find the perfect border-radius.”
This is more than a little patronizing. Delivering a polished, professional design and being flexible to changing requirements aren't mutually exclusive goals.