Interfaces are not two dimensional and static, they are three dimensional (time) in the sense that hey have to be interactive. Flat graphics are not interactive.
If you are going to just use photoshop, use a pencil, it's cheaper and easier.
People waste a lot of time iterating over pretty pictures and then pass all the hard work to front end engineers to sort out the cruft.
Worse still is when you then get a PM that wants the whole pixel perfect thing to jump through HTML5 hoops. With very little consideration to the time element of design, you are left with a lot of pretty images that you may as well start again with.
In browser designing is not an optional path to take. If it is going to end up in the browser it will have to be designed at some point in the browser, so designers should dump pixel perfect tools like photoshop as their main iterating tool.
Photoshop is still a pixel tool and getting old, Illustrator is now better for interface improvement IMO as it has pixel preview and can output svg too :).
CSS and HTML? A Designer does not need to know that to produce a design.
30 mb PSD vs someone who can code their design. Your pick.
I've did both: having a design than translation to HTML and people that could do both
There's an advantage to someone that will focus only on the design and another that will focus on only translating that to HTML/CSS/JS namely, that someone that can do both very well is rare
But of course, if they can do the design and a basic html/css of it usually can move the project faster. You will not have the greatest design or greatest html/css but it will usually arrive at a good solution