I would say both are important. But from my experience (and I’m not talking about web designers here), more people on this earth are artistic than logical.
A developer can make a okish design, it will work, it won’t look crap since they can copy ideas off others. No it won’t be anything new but it will be practical (which I have found is better than all flashy, all these flash websites with amazing graphics are just crap). But say to a designer can you code this application so it runs across platforms and only consumes X amount of memory and they will be stuffed.
As said before there is the problem of people don’t know of how much work a developer has to do, since they don’t see the code.
So in my opinion a developer is far more important. What’s better something that looks nice but doesn’t do anything or something that looks crap but works? I think a good example would be a car. The designer can make the car look sexy and wonderful, but it won’t move. While the developer can make the car move, but the passenger is just sitting on a metal frame with no external shell (or just a cardboard box they nicked from a tramp)