Here in my home country (Hong Kong), all air traffic controllers are required to obtain a PPL as part of their training. The reason being, if the controllers can better imagine a pilot's perspective, they can better accommodate and provide assistance to pilots. I don't think they are required to keep the license current, but sure they need to pass all PPL exams.
Is this requirement common around the world? (Note that I intentionally do not specify which country or jurisdiction. I'm looking for an answer which lists the countries which have this requirement.)