Methinks we are ignoring the elephant in the room. Sure, good teamwork will solve a lot of problems, regardless whether the designer can code or not. A team with all the skills combined can deal with both.

But there are people that have a vested interest in playing designers and developers against each other.

I've seen this happen time and time again, both via internal management and external clients: feeding designers and developers separately different bits of information (but never the whole picture), letting them make commitments based on that information and only then bringing the two together. The end result is that in order to make good on their separate commitments, designers and developers are left with very little room to compromise, which undermines the cooperation. The goal of this little divide-and-conquer game is to pressure them to come up with a result they otherwise would not have committed to given the time and budget constraints.

There's a reason why designers and developers are often very deliberately hired or managed separately for the same project. There are plenty of people who believe having designers and developers work together too closely is not in their interests.