Having the fortune of working backwards in my career, starting with fundamentals in Rails, to front-end development, and now to interface design and research — the clearest way I can say this is — if you’re designing for the web, you should know how to build for the web.
There’s a difference between being a production front-end developer, and understanding enough of the basics for how front-end works. I believe having enough perspective to inform and genuinally change the way you design is what you should try and achieve.
The other school of thought I agree with is prototyping in the tool or language that production happens in. If you, as a designer, can close the gap between your prototype and the language it’s going to made in — you should.
There’s so much less confusion when it comes to effort, scope, responsiveness and interactions when the designer can think like the developer.
Keep in mind this is around a specifically web focus. I’m sure there’s another argument to be made for mobile development and design.