Hello, D.J.J.
As was already said, nautilus is not the only application hidding its title bar
(and the close button along with it). Actually, nautilus adopted this only six months later than some others.
The main idea is hidding the titlebar, because it wastes space on the screen for no purpose
(the name of the folder is already shown inside the window
). This had the side effect of losing the close button.
The plan is to place the close button inside the window. Allan's blog post has some example mockups of how this would look like:
https://afaikblog.wordpress.com/2013/04/25/design-goings-on/
