It should be mandatory wherein the title bar is expected/desired by the user and not inherently the responsibility of each individual app by default. The wm should have the capability of drawing a titlebar for each app and do so by default unless the app opts to draw its own OR the user configures the system to not do so.
Yes, and that's how it's handled by the desktop environments I've used. Many programs have the option to use the system, program or no title bar. But should it be mandatory for every DE/WM or can the user choose one that satisfies whatever preferences the user has?
This way people like me still have the option to use window managers that don't have title bars. Title bars are useless for power users that know what program they're in and don't need them. To me they're in the way.
