To me it depends. If the propulsion problems suddenly appeared that is one thing. If the propulsion problems are reocurring because the cruiseline is to cheap the properly fix the problem it's another issue. Carnival is not the only cruiseline to have these problems. When I cruise on Infinity (Celebrity Cruiselines) reviews I read with some unhappy people who missed ports because of propulsion problems. These were back to back cruises.
However, on the other side of the coin. You have a cruise booked and the week before they have propulsion problems and call you to tell you your cruise is cancelled does that make you happy? The cruiselines are in a no win situation. If they cancel the next cruise 5 days or so before sailing and people have airfare these people are going to lose more then one port. The truth is things break, it's a fact of life. I'm sorry the OP missed the port but I think that is better then missing the whole cruise.
See I can't make up my mind on this issue.:???: I want the cruiselines to properly maintain the ships. However, I don't want it to effect my cruise. I only get one week maybe two weeks a year to enjoy a cruise. This year I'm not cruising at all. My next cruise is in May and I want the cruise to be wonderful. If they divert my ship from Mazaltan to Enseneda I will get off the ship long enough to buy a purse at Marios silver shop (I love that place) then I'm getting back on the ship and enjoying the fact that no one else is on board.