For me, it would depend on who's going and how much the trip cost. When DH and I took his parents to Alaska on a fairly pricey trip, we got insurance just in case somethign came up with one of them and we had to cancel. If it's just the two of us on a less expensive trip, I generally don't get it because it would be a rare thing for us to have to cancel.
Also, price out your insurance at different places. Often times it's more expensive to get insurance from the company the trip is with (like Disney or a cruiseline directly). Try insuremytrip.com or csatravel.com.