While I compleatly agree that web designers need to understand the medium they are working in and be familiar with the limitations, that doesn’t necesarily mean that they have to code. Similaly UX designers need to understand what interactions are possible and what functionality can be produced, but does that mean they need to code PHP or JavaScript? While it would be beneficial, i think it’s perfectly possible to understand a medium without having the technical skills to deploy in that medium. For instance, how many architects do you k ow that could actually build a wall, let alone a whole building. Yet they still understand buildings as a medium. So I don’t think your problem is with “designers who can’t code”. I tinker it’s with bad designers full stop.