I always buy
travel insurance, but not usually via
DCL. When cruising I consider it a necessity due to being out of the country and the potential need to be evacuated & flown back in should some random accident occur and seriously injure someone in our party (you never know until it happens).
DCL's insurance is fine but pricey. I use a good plan that I can get due to being a USAA member, but you can ask and shop around. Read reviews & make sure the insurer actually pays out the way they're supposed to if a claim has to be made & without an unreasonable hassle (I know my insurance does because I've had to file claims with them before, but you should definitely check reviews for this as there are some bad apples out there).
I like to get a plan that pays 100% for covered reasons (illness, etc.), but also will pay 75% if you cancel for any reason at all: this is called a Cancel for Any Reason plan. They're more expensive & you have to buy them within a few weeks of booking the trip, but imo they're worth it. You never think you're going to cancel a trip until the need/want arises later- and that need/want is not always a covered reason, so a CFAR plan at least gets you most of your money back in that case.