>I never understood why people are buying proprietary, expensive stuff like Photoshop just for mock-ups.

People have been using Photoshop for years (like myself), and personally these people feel that open source equivalents like GIMP are still way off in terms of competing with Photoshop.

>First of all, vector graphics tools like Inkscape are a lot better suited for that task.

I have used both Inkscape and Photoshop for quite a few years. It's unfair to even compare them. And, frankly Photoshop _is_ far more powerful if you want to go for high fidelity mockups.