Some random doodling about the possibilities of using the functionalities of an OAI repository as an API lead me to information about metadata repositories, harvesting and edna (Education Network Australia, ' Australia's free online network for educators').
In linked document, Harvesting Overview, they state: "In addition, the edna search API is embedded into numerous other websites – providing access to the edna repository and indexes from external websites. The benefit to you is that, by providing your metadata records for harvesting by edna, you increase exposure to your valuable education and training related resources."
It's a good summary of the processes involved in setting up an OAI repository for harvesting and of the benefits for the organisation; including increased visibility of resources, maximising return on investment [ROI] for created resources and associated metadata and benefiting from services such as RSS that can be delivered back to the organisation.