The Room of Requirement was a strange thing. It was more than any wizard should have been able to do and no one knew what it's purpose was. Perhaps that was because it's purpose was already being fulfilled.

Dumbledore hires a Defense Against The Dark Arts professor with plenty of job experience training teenagers to fight monsters and Dark Lords. In fact, Harry's fifth-year professor may be more than a little overqualified. Or overkill. Or both.