Add yet another reason. Designers who can code know the cost of the pretty frills they add to their designs, and so naturally can select pretty designs that will be easier to actually render in HTML.
Understanding what you can do in CSS and what will require an image file or javascript to work is pretty important in designing an efficient site. I've seen designers with pretty looking ideas that are really difficult to implement. They say, "but you can do it right?" and it can be done, but they don't realize what they are asking for.
1) Considering the costs of implementation limits the space of possible designs thus reducing originality.
2) Considering the costs of implementation limits the space of possible designs thus inspiring creation.
Which is just an obnoxious way to say that sometimes it's pretty damn useful to design things that are hard to implement. If you're too tied to the CSS, you won't try dumb enough things often enough and that's a direct line to insipid design.
But when there are two designs that are equally good, start with the easier to implement one. If you know how to code, then you'll have a sense which one that is. If not then you'll consistently make things unnecessarily hard to do.
And at this point, I agree. If you know how to code you can use the principles of practicality to guide your final decision to a design that fits the medium.
But I'm always starting with pen and paper drawing things that could never exist in your browser. I think it's integral.