Should content strategy be included in your UX deliverables?
Yes, with a couple of conditions.
I have often lobbied that content strategy and "tone of voice" should be a part of any UX architecture, and that type of arguement comes natural and easy for me because… well, I'm a writer. My background is in journalism, and so I have experience in writing that many people in UX don't. So my first disclaimer is don't just have content strategy because you think it's important or necessary… only have it if you (or someone on your team) are capable of creating one. Content is INCREDIBLY important, and you can have the best interface in the world that fails because the content that is a vital part of the experience is badly done.
Second condition: Developing a content strategy, like designing an effective interface, involves knowing your user. Creating content or defining a strategy where the information being presented is targeted at Gen Y and the majority of your users are ages 40 to 50… well, that's bad. The tone of voice has to be appropriate to the audience, as well as the context of what is being designed and delivered.
So, in summary: only do it if you can do it well and have the foundational understanding of the users to target the content strategy appropriately.