When a 6-inch destroyer becomes a cruiser

Are 6-inch guns on destroyers ever a good idea?

Not for interwar or World War II destroyers; on very large modern destroyers they can fit, but those ships are effectively cruisers.

A historical boundary separates the compact destroyers of the 1920s-40s from much larger modern surface combatants. A 6-inch mounting would impose too much weight and volume on an interwar or World War II destroyer. Today's very large destroyers can physically accept a twin 6-inch weapon. Once they reach that displacement and carry cruiser-scale guns, their capability and mission make them functionally closer to cruisers regardless of the official label.