If you are any type of web based application than the app and marketing are intertwined and should not be separated, at least in terms of appearances from the outside. That said, when I was doing this type of work for companies as a consultant, we many times built out a Wordpress based marketing site and tied it to a custom developed application. It was seamless to the user but allowed the business team to add and update content without getting the dev team involved (or hire it out easily). In fact many times I would hire a wordpress agency to build out the template and initial marketing content while my team worked the custom application. This also allowed us to deliver and have marketing, email collection etc all setup well in advance of the app launch.

Only time it is acceptable to outsource your marketing site is when your site is not part of your product. At that point, it doesn't matter if it is outsourced for development and maintenance, as long as things are getting updated timely.

There is a strong argument to be made that even in an organization when app and marketing are somewhat to fully integrated that not the same teams are working on them, since the two end goals are generally quite different and have their own timelines. e.g. you don't want your developers who are fixing issues and developing features for clients worrying about the fact you need to replace content or update the look of the blog.