Photoshop's power = macros and actions (IMHO there are commercial outfits producing actions for Photoshop)
I have yet to see a web based design tool that has a good macro system - even basic ones (like auto align, increase/decrease font size, etc.) are missing
It's like telling someone who uses emacs to go and use gedit.
Obviously power users will be hard to make switch, however there is a whole lot of people paying for "less powerful" code editors because they have a much lower barrier to entry and do the things way want to do better, bringing up textmate vs emacs at least to me strengthens the argument.
I have yet to see a web based design tool that has a good macro system - even basic ones (like auto align, increase/decrease font size, etc.) are missing
It's like telling someone who uses emacs to go and use gedit.