Building responsively is an investment for the future. You can bet good money that there will be more and more diversity of screen sizes as we keep going.
I'd be interested to learn more about their design and implementation workflow, that they mention RWD taking 'almost twice as long'. How much time are they wasting building static comps that illustrate a picture of what some people will see when they come to their website? This is speculation - I don't know how much live prototyping they do in their design process.
I'll have to mention Edit Room again, my web design tool that makes creating responsive, multi-screen web layouts as easy as dragging things around on-screen. If you start your design work in the browser, you save so much time that lets you go that extra 10% on a design. That gives you the time you need to make your designs work everywhere.