I can think of another very good reason to hire a dev shop: you already have a developer or two, but want to try out a feature that's well outside of their area

I can think of another very good reason to hire a dev shop: you already have a developer or two, but want to try out a feature that's well outside of their areas of expertise. For example, your core application is a Rails-based website, but you want to try offering an iPhone app. Note that if that feature is successful, you will probably need to train your developers in that technology or hire someone, and rewrite it from scratch.