I guess it depends. I buy it because my health insurance will not cover anything outside of the US. One trip to the health center will cost more than the insurance for the family.
My logic is that I could afford to lose the cost of the cruise; I wouldn't be happy, but I wouldn't be in serious financial trouble either. What would be more difficult could be a large medical bill. I've seen a teen on the ship fall and sustain a compound fracture, medical evacuations for various issues, etc. None of these involved my family, but they could have.
In addition to the medical coverage,
travel insurance will give me an amount to spend on clothing if my luggage is lost or significantly delayed, cover my out of pocket expenses when the ship can't leave due to a hurricane, or cover the cost of the trip if one of my elderly parents becomes suddenly ill, etc.
If I go to WDW, I don't buy insurance. My logic is "worst case, I'm out part of the cost of the trip." Parts of it can be deferred/canceled at no charge, etc. I'm in the US, so have medical coverage. If I go out of the country, I buy it..mostly for the medical coverage.