"Desert Island Discourse" is talking to yourself in your head. This helps people be most themselves, and it leads to very voice-ful writing, and a kind of solid, rich centered but unrigid sense of identity. In contrast, collaborative writing makes it harder for people to use their "gut voice"--or "personal voice" or "personally connected voice. Some writers of "open essays" such as Montaigne, White, and Anderson--the "musers" and "deep explorers"--tend to have very deep, idiosyncratic, strong, and interesting personal or gut voices.
I guess I connect this to the fact that having a dialogue with yourself is the most personal thing in the world to do--and that people who learn how to do it become more themselves. My point is that it's a wonderful and deeply human thing to be able to do--to learn to talk to yourself as though you were on a desert island--and that it makes you more yourself. This is a good angle on the disputes about voice and self. That we get to be more ourself and feel ourself more our self--the more we can have conversations with ourself--allow voices in our head.