I'm the opposite: for me extra or bigger screens are overrated. I'm definitely the minority because almost all my colleagues have a second screen. I think DHH is the only one I have seen using a 13" because of the focus. And I agree with him: one maximized window at a time, all my focus in it. No neck craning etc. And I can work from anywhere, don't have to be at my (home) desk. I organize projects by desktop (Mac OS feature) and switch through them with 3 finger swipe.