Fortunately we have never been inconvienced by not getting to the ship on time but we have been witness to others less fortunate. On a cruise from Vancouver to Hawaii approximately 70 passengers coming from the Chicago area were delayed getting into Seattle because of weather and missed the departure. And they couldn't get on at the next port because that was five days later in Hawaii. After this incident my husband and I vowed that we would always fly in at least one day ahead of time. On our last cruise to Alaska quite a few passengers experienced flight delays. Two ladies at our table made it to the ship on time but their luggage didn't. It was delivered to them at the next port which was two days later so had no formal clothes for the first formal night. They were so stressed out onboard the first night from the the worry of not making it to the ship on time that I really felt sorry them. What a way to start off your vacation.
We were in Portugal last week and flew into Lisbon via Amsterdam. When arriving at the Lisbon airport we were advised that many pieces of luggage did not make the flight. My husband had witnessed large bags of postal mail being loaded onto the plane in Amsterdam so we are guessing that our luggage got bumped by the mail (due to weight restrictions). So unless you're flying direct there is always that worry as well.
Plus if you get to the ship early and can sit around the pool if the weather is warm, have a drink, etc., it makes it seem like you have an extra day on the ship.