I think, have notions in both domains, but do not try to be the best at both domains, sure, some people can successfully take both tasks (sometimes more) at once, but it is a risky thing to do.
Designers can see and foresee rules, environnements, they can modify them and adapt them to have a pleasant result, but it is creation, there are rules, but not set in stone, this is more art than science.
Programming however is application, doing the core, implementing and make a machine interpret the result, while keeping in mind that organisation and restrictions are part of the deal, this is more science than art this time.
This is my personnal interpretation of both, there are of course more accurate definitions, but I think you should be open to both, while working in one side in particular, could be wrong though.
I know it is not a very useful awnser, but the question's awnser is "you can, but because you can, does that mean it is better?"