Those are rare songs long sought after by fans. It's great that people have a chance to hear them at last. According to Nick Pegg there is a lot out there. We get spoiled periodically too like when Toy was leaked as were The Leon Tapes. You even have funny stuff like this.

https://m.youtube.com/watch?v=NrtXFTw2ico

Personally I wish for something like the Dylan bootleg series coming out of the Bowie Estate. It doesn't hurt the legacy. His core body of work is so strong.

Unreleased stuff is like seeing sketches and works in progress. It only adds to the layers. Just because you have heard a demo of Space Oddity it just adds a new dimension to the final piece.