Dan Mall raises the most important point with these questions. Use the tool that allows you to best deliver a result for your client in the timeframes you’ve got.
Sure, you could get this same level of approximation in code instead of Photoshop. In fact, my friends over at Sparkbox make what they call a Style Prototypea staple in their process. For me, however, it’s back to the quickest method of expressiveness. When I’m writing code, my brain isn’t in creation-mode; it’s in debug-mode.An excerpt from Question About Using Photoshop in Responsive Workflows