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.
web designers absolutely should use photoshop, because they are essentially providing the "theme" around the wireframe, that may be designed by the UI designer.
It depends on the specific design. If it's wordy and document-like (eg: http://www.w3.org/TR/html5/) then it may be better to bypass Photoshop and start writing the HTML/CSS, since Photoshop is ill-suited for mocking up those kinds of designs.
On the other hand, if it's more graphical (eg: http://www.ubuntu.com/desktop) then I'd agree that Photoshop is absolutely necessary.
And no, I still disagree slightly. It's not about the kind of design the final product is, it's about what function your role is fulfilling. Are you designing the way the people interact with the application, the way the application looks and feels, or both?
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.