It is mostly about popularity, being a standard, and so on. If you invent your auth, you don't expect any http library to support it, but you expect almost all http libraries to support basic auth. Now Microsoft NTLM lies somewhere between. You can expect some libraries to support it in the core, and some apps to use the customization capabilities of the http library (such as adding header and cookies) and handle it themselves.

That's the way I think and decide how to vote something in and/or leave something out.

With cookie auth mode, my ideal would be that the restclient to add cookie jar support in this mode, but the application needs to handle the auth call since the auth call is usually an app level call. If I were to find an use a rest library, that's what I would expect it to do I guess ...