You really have to decide what you want insured, how likely you're going to need it, and what current coverage you have. Many US insurance companies do not insure you when you leave the country, which would include a cruise ship of foreign registry. If you get sick or injured while on board or on shore, you would not be covered. Check your policy or speak to your agent, you may be covered for this. If not, the big question here is how likely are you to get sick or injured? You probably know how often you get sick, and how active you are going to be, so you may be able to make a good guess at this.
The other part of
travel insurance generally covers lost lugage, trip delay or cancellation. Again, how likely is this to happen? If you have to cancel because of a family emergency, this will keep you from having to eat the cancellation penalty. If your family is in generally good health, you may decide that it's unlikely you'll need this. If you're delayed and miss the ship, you'd be covered, but if you're flying in the day before, this is pretty unlikely. Lost bags happen, but again, not very often, in my experience.
In the end, you have to judge how much the peace of mind of not having to worry about any of this is worth to you and do what you feel most comfortable with. The cost is usually around 3% of the cost of the trip. You can get coverage for less than the blanket $99/pp
DCL charges.
My take is that I'd rather have it and not need it than the other way around. I once saw somene slip on the deck of a catamaran during an excursion at Key West and split their head open pretty good. I don't know if she missed the rest of the cruise (this was on the first full day) or if she had insurance, but I would suspect that she never figured that this might happen to her.