I usually do buy the insurance if I'm using more than 1000 or so airmiles in total (don't really know how or why I arrived at that number, but everyone needs rules and I guess this is one of mine)
Anyhow, in November 2000 my DW & I were to travel to NYC for me to run the marathon. 3 days before leaving our DD contracted the chicken pox and to make matters worse my wife hadn't ever had them. Fortunately we had purchased insurance and they reimbursed us for her flight, which obviously she couldn't go on.
They offered to reimburse for my flight if I wanted to cancel my flight as well.