If you live in the port city, I probably wouldn't buy cruise insurance, but otherwise I would. If you wind up getting a flight delay, etc., the cost of the cruise insurance could be well worth the money. We always leave a day early and plan to go back a day late just to avoid that problem, but you just never know. Also, you never know when someone will get sick and not be able to go. It would be a bummer to lose the money you spent if someone got kidney stones or a broken foot a week before the cruise.
We buy our insurance through CSA and I think its a good deal.