The fact that I work within the framework of a presupposed external reality doesn't mean I believe in that reality. I function according to that reality because I know the consequences of acting otherwise can be painful. Occasionally I do something crazy to "test the boundaries," but the patterns of "my reality" remain consistent and so I continue to act accordingly.

Whether it's useless to think this way is arguable, but irrelevant. I don't think usefulness has any bearing on truth.