Personally I loathe having to use any sort of GUI because in general its slow and I cannot as easily automate tasks to customize my workflow.
There's a lot of research out there that provides a lot of evidence that GUIs in general aren't slow, assuming they're well-designed; they feel slower to people used to CLIs and keyboard-driven interfaces, but when you actually measure objectively with a stopwatch the results tend to favor GUIs.
I think it's the second part of your statement about automating tasks where CLIs really have the advantage. For GUI programs to be automated they have to provide a scripting API much more complicated than just an input and output stream -- and then, of course, you need a scripting language to bind them. OS X's Automator is a really great glimpse of how that could be done in an entirely GUI fashion, but it also shows the limitations of such an approach -- most of which aren't in Automator itself, but the very fact that applications need to be written to support it. (Of course, some of the built-in actions let you run AppleScript or any shell language programs.)
There's a lot of research out there that provides a lot of evidence that GUIs in general aren't slow, assuming they're well-designed; they feel slower to people used to CLIs and keyboard-driven interfaces, but when you actually measure objectively with a stopwatch the results tend to favor GUIs.
I think it's the second part of your statement about automating tasks where CLIs really have the advantage. For GUI programs to be automated they have to provide a scripting API much more complicated than just an input and output stream -- and then, of course, you need a scripting language to bind them. OS X's Automator is a really great glimpse of how that could be done in an entirely GUI fashion, but it also shows the limitations of such an approach -- most of which aren't in Automator itself, but the very fact that applications need to be written to support it. (Of course, some of the built-in actions let you run AppleScript or any shell language programs.)