Earlier quoted context omitted.
I think what unavoidable is saying is that photoshop, specifically, isn't a necessary step. Personally, I hate mockups in it. I try to use paper and pencil, if it needs to be formal, I use balsamiq. I do agree that you need SOME sort of mockup, but I actually think low res mockups are more effective because people aren't distracted by the fact that an underlined link in PS doesn't quite look right.
thats not a mockup thats a wireframe web designers absolutely should use photoshop, because they are essentially providing the "theme" around the wireframe, that may be designed by the UI designer.
On the other hand, if it's more graphical (eg: http://www.ubuntu.com/desktop) then I'd agree that Photoshop is absolutely necessary.