I sometimes do and sometimes don't. We both have extremely good out of country medical coverage through work which would always be our primary travel medical insurance so really I'd be booking trip cancellation policies. Land trips and cheap cruises I don't bother as land trips I can just cancel my hotel and pay a change fee for the air, cheap cruises, I self insure. Once the cruise level goes over a certain amount, I buy cancellation insurance.
Have never used insurance. We did lose $1,000 to Princess once when we decided to cancel a cruise after final payment because United messed up our flights so badly we were not prepared to do the routing they offered. That would have only been covered if I'd have bought the extra "Cancel for any Reason".
If we didn't have such awesome medical through work, I wouldn't travel without insurance.