Look at it this way:
average cruise cost: $1000
average insurance cost: $100
If you cruise 10 times and never use it, it was a waste of money.
But hindsight is always 20/20
In all seriousness- I normally never buy any insurance for products I buy, but I always buy cruise insurance because 'you never know' what can happen between when you book and sail, so it's good piece of mind if nothing else.