Bruce McCarthy at User>Driven contrasts the roles of product management and product marketing. He writes:

The most successful organizations I've seen have a split between product management and product marketing. They have people who support sales and others who set product direction. Both talk to customers every day, but the conversations are very different. In the end they work together, of course. The work of each feeds into the other. But in the end, the mission of one is to bring the message from the market in, and the mission of the other is to get the message out.

I made the same point in Product Management Job Responsibilities: A PM listens to the market; a PMM talks to the market. It is indeed a different mindset and a different skill set.

When gathering requirements for the product, product managers and product marketers should interview customers together. One is looking for problems to be addressed for product users while the other is seeking to understand how to communicate to product buyers. As Bruce implies, there’s inbound marketing and outbound marketing. Inbound brings requirements in to the building; outbound takes the resulting products out to the market.

(However, I wish that Bruce hadn’t characterized outbound marketing as sales support. While outbound product marketing communicates to the client base through the sales channel, we should be focused on the market full of customers rather than on customers one-at-a-time. Supporting sales people is the role of sales engineering, not product marketing.)

Market requirements and positioning documents are owned by those who best know the market. Product management knows what to build; product marketing knows what to say.