The obvious answer is, use them. It won't kill you. But consider this situation. It happened to one of my friends (and might happen again). What would you have done?
Years ago my best friend had to go to Washington, DC on business. She had one night free, and was faced with the bewildering yet very happy decision of whether to rent a car and go to the Inn at Little Washington, or eat in one of DC's incredible local restaurants. I was pushing for Roberto Donna's Laboratorio. And then her father-in-law gave her a gift certificate to a bland, boring chain restaurant. She didn't want to be on his bad side forevermore, so on her one precious night free she ate at that chain. She could of course have given the certificate away. But then he would have asked her how she liked the food. Should she have lied? Or told him that she gave the certificate away to a friend who really liked that chain? He would have been offended by the latter, and guessed that she didn't want to eat there. So what should she have done?
Two other questions. I've always been embarrassd to use gift certificates at a place where I'm known. I have the nagging feeling it makes me look cheap, or sends the signal that I can't afford to pay. And what if the certificate is for an amount far too large (or small) for the meal I want? Most places don't give change.