I agree to both of you.
@elextr: of course everything should be on an voluntary basis. If a developer does not want to share the code then ok we have to accept it. It might also be a matter of time to write code in that way that it can be commonly used. But also like @codebrainz mentioned I would expect that most developers have a natural need to prevent code duplication and also to use existing code if already available.
So hopefully, once started, it would grow with the time.
@codebrainz: yes, I also see it as a common lib specific to geany plugins. Not comparable to general purpose libs like glib or gtk.