At my current company, the marketing department has been using agile for years. I think the challenge you may be encountering is that so many people equate "Agile" with "Scrum".
Scrum is great. Great when your organization can tolerate not changing the backlog for an entire sprint. Even with week long sprints, this can be a challenge for an organization like Marketing.
Enter Kanban or even ScrumBan. These flow-based models offer models that allow marketing to respond to rapidly changing backlogs while still giving a framework that provides structure in an agile-based mindset.
Funny enough, Agile coaching is a similarly difficult to fit into "Agile" if we think of agile as only being Scrum. I'm working with a five-person coaching team and we use ScrumBan to manage our coaching engagement with the client. We went with ScrumBan because it gives us the planning window and an ability to get an idea of our capacity and goals for a week (1-week sprints) while still being able to respond to the clients "I need it now" requests.