If you limit your designs to what you feel comfortable coding, you shouldn't be doing the coding.
But here’s the thing: I was never a coder / programmer / developer / engineer / insert technical title of choice here. I was always, first and foremost, a designer, a UX'er. Coding was a way to get things built when no one else was available, but it was never my passion, nor an area of expertise for me.
Often, however, I'd run into the same issue: at some point in each project, I'd start letting what I felt comfortable building dictate what I would design. Even if I thought a particular design solution was best, I wouldn't pursue it if I didn't think I could build it. I'd pull back from things that seemed right conceptually, but which I knew would be impossible for me to execute.
As front-end development became a lot less about markup and a lot more about engineering, I'd start thinking about my technical limitations in the earliest stages of the design process. This resulted in my killing off many ideas before they had a chance to prove (or disprove) themselves. It was like an architect deciding against a pedestrian tunnel because she wasn't confident in her ability to operate a tunnel boring machine.
This issue — design ideation being restricted by the designer's technical chops — is one of the primary reasons I don’t believe designers necessarily need to code. Eventually, even the most technically proficient designer is going to forego a viable-if-technically-complex design solution simply because they can't build it.
As I've written before, designers must be familiar with the technology underpinning their designs, and they should even be able to sling a little code — for fun, if nothing else. But I remain skeptical that a top-flight designer can also be a top-flight developer. Even if someone is extremely competent and confident in both disciplines, chances are that — on some level — they'll start thinking execution when they should still be thinking concept.
Aside: I find it interesting that this debate typically focuses on designers taking on engineering responsibilities, but rarely do we hear that engineers should be more like designers. The implication seems to be that designers have some special talent they should supplement with a core set of fundamental technical skills. I think this line of thinking is grossly disrespectful toward the engineer's specialized creativity and talent — not to mention years of training and experience.