Stephen Downes, widely recognized as one of the original inventors of the "MOOC" concept, on why courses should be open: "MOOC - The Resurgence of Community in Online Learning".
Why make our courses open? Think of a course as like a language. If a language is closed, it dies. If people are not allowed to speak it, it dies. To enable people to genuinely participate in the culture of a discipline, whether it be physics or chemistry or political science, the content and the materials of the discipline must be open.
There is the danger that a cultural or linguistic group will retreat into itself in the face of this risk. I look, for example, at the state of publishing in communities like Finland or Sweden, and find that open access is very limited, as the publishers imagine that there is no other place for Finnish or Swedish speakers to turn. But they do turn, as we know, to open online content in English.