Meh, people who are Photoshop gurus can change their layered whatevers prolly just as fast as a CSS guru can change their stylesheet.
Since I now usually have more than one stylesheet (mobile and SomethingBigger), it's no longer as fast as it used to be for me.
Most people who sign off those sites need to know how things will look like.
This definitely comes after they know what they're putting on the page, why they have a web page, what their visitors are supposed to be doing on the web page... they have to sign off on that stuff first, or the Photoshop guy is wasting his time.
They would not be able to figure out how things will work and how will look on the website without the PSD mockup.
While some companies use a full-fledged PSD for architecture, it is not some necessity. Some companies use wireframes (sometimes created in their own large, complex software) or even whiteboard mockups. It wouldn't surprise me if the technology used for decided what visitors do where and where things are placed is determined purely by what kinds of people they've got working for them (a UI architect? Or a graphic designer? Or someone else?).
It is like when you're selling a house which is old and maybe not in a good condition.
I realise developers like sanamine have to try to imitate static, inflexible raster-pixel images with their web pages, and I feel very very sorry for them, mostly because to make things actually work for people they need to work twice as hard as someone who can design for the device and for people, instead of for someone's static canvas dream.
I know I'm privileged to buy jeans that fit me, rather than being given jeans and told to stuff my butt into something too small or wrap belts around something too big. I'm lucky I can build what's important (functionality, flexibility, people-oriented) instead of needing to pretend the web is a billboard and cater to some artist instead of catering to the ones we should be catering to: users.
The artist should be catering to users too.
I suppose it would really help matters if some graphics company could make a graphics program that's sorta like vectors, except flexible like web pages. Made specifically for graphics people who aren't going to learn HTML and CSS and don't want to. I mean, the stuff out there for specifically print is astounding. The canvas would be a pretty good imitation of a browser, with the ability to show the graphics guy the possibilities you get cross-user, cross-platform, cross-browser and cross-device.
Likely Adobe will make it and it'll cost $4000 or something... :S