I don't think this is true at all. The best designers I have worked with wouldn't know an HTML tag from a line of JavaScript. They are two completely separate s

I don't think this is true at all. The best designers I have worked with wouldn't know an HTML tag from a line of JavaScript. They are two completely separate skills in my opinion. I think a lot of the confusion comes from the ego involved. Designers see developers actually building things and it can create a feeling of inadequacy. They forget that developers have the same feeling of amazement watching them come up with some beautiful new design. I would much rather work with a fellow specialist at the top of their field, helping them to realize their vision, instead of battling over implementation details with a generalist. Anyone who claims to be equally good at both is self deluded.