Yes. It is worth it - especially if your Credit Card or your standard insurance does not cover your trip. Murphy is always around the corner. You can easily spend over USD $10,000 out of your own pocket for transportation and medical expenses.
Some cruise lines will require you to buy an insurance policy from them, however, make sure that you read the fine print. A cancellation insurance policy, in most cases, is not extremely expensive and worthwhile considering that most holidays (flights & cruises) require you to pay in full before your tickets are issued.
If you cancel your holidays at the last minute, for any reason, including medical or any last-minute challenges, you might lose the whole fare.
Pay by Credit Card, if you have one, as most credit cards have some sort of cancellation insurance. Be sure you check your credit card policy.
Some cruise lines will allow you to cancel or move your cruise a certain amount of days prior to departure. Be sure you read the terms and conditions - most cruise lines will not refund your deposit in the case of cancellation, but the insurance might do (you will not get a refund for your insurance premium).
Sickness, an accident or medical emergencies can easily happen to any traveller, at any time, on any type of holiday. Travel insurance is not expensive compared to the value and peace of mind that you get out of it. Make sure you include all pre-existing medical conditions, as ignoring this will cost you dearly.
Tip - instead of cancelling why don't you enquire about the cost of moving your holiday.