I have lost track of the number of times I have heard that argument, and implemented the API only to never have other API clients, or the API clients that do come later have very different needs and I need to create another API for that. I don't think API re-use is a strong argument unless the other client will be consuming the API imminently. I've heard the of "course we will have a native mobile app" argument enough times to often identify it as wishful thinking. Many clients are often satisfied with with a good web app.