The problem is if there are problems with the flight. The cruise leaves with our without you.
Just yesterday, my DH had to fly to Montreal for a business trip. About 5:00 p.m. he walks in the door of our home! His flight took off, after 15 minutes experienced engine problems, they turned around and landed where they took off. Then they couldn't get him on a new flight until today (24 hours later).
Luckily, he could just come home, and the conference is still there, and he just missed a day of sessions. Had it been the cruise, they would have left without him.
We flew in the day of the cruise our first time because Disney booked our flights and they booked it for the day of the cruise. Since then, we have booked our own and we have flown in the day before. We are more rested and more relaxed by doing this.
Anyway, the problem really gets down to the fact that the ship leaves. If you were just going to WDW and you missed a flight, it would still be there the next day.
Of course, statistically speaking, the odds are in your favor that you will make it alright. On our last cruise, one of the DIS couples that came to our meet had just made it to the ship. They had been stuck on a runway for hours waiting for a problem to be resolved. BUT they DID make it. So all was well.
Good Luck! Hope you have a great trip!
DJ