I think it is because you get most of the princesses at the breakfast and it saves time from having to wait in a very long toon town or ariels grotto line to see them.
For my family dining with the princesses was totally worth it. We were on the DxDDP, so didn't have to worry about OOP, but we would have done it even without the DDP. It saves so much park time when you can see the princesses at your table rather than catching them in the parks. I can't comment on Akershaus breakfast as we did lunch, but the lunch meal was very good and the interaction was wonderful...Ariel spend so much time with DD3 when she told her she was her favorite princess (and was dressed as Ariel).
CP is good too. We dinned there several times for lunch and once for breakfast.