In my experience it’s important for designers to code. I don’t really believe that it’s that different than a painter actually putting paint to canvas. The heart of the debate I think, is what constitutes “done” - is a design done when it’s on paper/psd? Or is it finished when it’s an html page. I don’t believe that any creative work is finished until it’s actually in the medium it’s meant to inhabit. A painting is not done until it’s in paint. A sculpture isn’t done until it’s in marble. A psd to me is an idea - albeit a very detailed one. If there’s something the startup world harps on, it’s that the idea is less important to the execution. 

One idea in here is that the duo of the modern era is the programmer/designer pair. That is absolutely gold. I have done almost all of my best work when paired with a designer.