We've done several cruises with the same itinerary but different shore excursions. One of the reasons we continue to enjoy the excursions is because we have a better idea of what we want to see and which excursions are just a waste of time and money. There's always something to enjoy no matter how many times you visit a port.
On our first cruises we were so excited, we had to be the first in line to get off the ship. Sure, we were cruising...but we were running!
For example, we noticed we never leisurely just enjoyed the view and ambiance of being in port. Well...with a repeat itinerary we don't feel that 'rush' to get off the ship.
So for us, a repeat-itinerary has been a good thing. We're actually enjoying the cruising experience more.