Prior to the Disney Fantasy - Hurricane Sandy cruise, I would have said that Disney would always err on the side of caution, with guest experience being the bottom line when making these types of decisions. Sadly, I no longer believe that. I believe that both Carnival and Disney will make the decision that is most favorable to them financially. I think Carnival knew they had engine issues and took a chance that everything would be okay because they didn't want to cancel a cruise. Unfortunately, Disney took that same gamble with the Fantasy last fall.
I was on a Disney Magic cruise several years ago (2006?) that had engine issues. We ended up not leaving St. Thomas until a part and a person to install the part were flown in from Norway (I think). We finally were underway about 2:30am. Due to that delay, we missed Castaway Cay. People on the ship were furious and we ended up getting 1/2 off a future cruise. I personally was thrilled with that offer! My concern is that if that identical situation happened today, the Magic would have left St. Thomas and potentially had a problem while at sea. If that had occured, we would have been in the exact same situation as the Carnival ship.