It is good for reducing stress, etc, and it has been proven that stress can be significantly detrimental to health, so yes, laughing is good preventative medicine. If you are already suffering health problems, or depressed, then it probably won't have much of an impact.
I think even it is therapeutic, it's temporary. It's like eating chocolate, which releases all the good neurotransmitters, and when the effect tapers off, reality kicks in again. Not sure if it actually cures.
It is a great stress reliever and releases all sorts of feel good hormones into your body. Sometimes we just take ourselves too dam seriously. When we can share ourselves with a giggle we give others permission to be less up tight.