Is Montenegro worth adding to a Croatia or Balkans trip?
Montenegro is worth adding when you give it more than a border-crossing cameo. Kotor alone can impress, but the country makes more sense when the bay, coast and mountains have room to connect.
Answer
Montenegro is one of the easiest Balkans add-ons to justify because the landscape changes quickly: bay, beach, lake, canyon and mountain sit close enough to tempt you. The trap is treating that compactness as if it removes travel friction. A Dubrovnik-based day trip may be satisfying if you only want a taste, but it will not show why Montenegro is more than a pretty old town. Give it a few nights and the route starts to work: Kotor Bay first, then either coast, Lake Skadar, Lovćen or Durmitor depending on the wider trip.