Actually it might be good to focus on what you do best (up to a point): If your a good coder, make an awesome product. If you are an awesome marketeer, use marketing to your advantage.
I think that the problem lies in busywork instead of creating value, not in doing what you do best (and if you do that, good chance that you can find people who are good in waht you need that will join you).
You are spot on about busywork and creating value.
I agree that founding teams should try to get complementary skill-sets in, but this can't be used as an excuse.
Or differently put… while I agree that everyone should do what they are best in, "focus" should come a bit later. Especially in the early phases founders are cross-disciplinary. I see a few people using the "focus on what you are best" as an excuse for just "doing what you like most" and ignore/hire/blame the rest.
I think that the problem lies in busywork instead of creating value, not in doing what you do best (and if you do that, good chance that you can find people who are good in waht you need that will join you).