I see that a user can subscribe with the CMC Registration plugin, but there doesn't appear to be a way to unsubscribe with the same plugin.
Ideally, when a logged-in user edits his profile, the plugin would recognize whether he's already subscribed to the list that the plugin is configured for and the checkbox would be checked or unchecked accordingly. Then, when a user saves the profile, if the box is checked, he'll be subscribed, and if it's unckecked, he'll be unsubscribed. Does that make sense?