A Testy Copy Editors thread turns to an interesting subject: Should aspiring copy editors learn how to design?
It doesn't hurt. But I find myself agreeing with the folks who advise working on copy editing skills if you want to be a copy editor. Knowing how to design will make you more marketable. No question there.
It all depends on your priorities, though.
If you need a job above all else, you're better off knowing more. That's obvious. But if you have the luxury of some time, and words interest and excite you above all else, be selective.
I'm not in the business because I'm a designer. I'm there because I enjoy working with the language. If I had to learn design, I would do it. Probably even enjoy it. But it's not my motivation for the job.