One of the scenarios we need to support is where you have a team of developers collaborating. They may not be working on the same physical instance of Node-RED, but they want to be able to easily share assets (ie flows/subflows) between themselves.
Putting the library under git control is certainly one way of doing this - and something I'm looking at for the library overhaul in the roadmap.
You can certainly do it manually today. But what you don't get is any automatic updating of what your deployed flows are using. As I said in my original reply, once you copy something from the library into your deployed flow, you lose that link. You still have to manually update your deployed flows to any such updates incorporated.