One of the legacies of my time in the service is that I do nothing without a plan. Also, I'm an obsessive-compulsive organizer, so I enjoy planning probably as much as Joely Sue loves to cross-stitch.
Yet before I get into anymore marketing stuff -- and it's in the works -- take a look at what Tam says about the most important marketing tool of all. She's right, too. On every writer's business plan, the #1 item should be story. It's the center of publishing; without it our universe collapses.
As to whether a writer should deal with marketing or not, I think it's up to the individual. It's been my experience that a certain amount of targeted, unique marketing by an author sells a lot of books. It's been my observation that 90% of the marketing which most authors presently engage in actually does little to nothing for their books. Your mileage may vary.