A cummerbund is designed to increase intra-abdominal pressure, which stabilizes your entire midsection I think. A bit like a weightlifting belt.
Weightlifting belts are bad for you unless you are actually competing. When lifting weights you want your abs to develop too! The belt tends to weaken your abs because it allows the belt to take much of the load. Lifting without a belt develops great abs.
As a physical therapist I am well aware of the consequences and risks of using a weightlifting belt. It's good to hear that the myths about these are dissapearing.
I do still think there is a good reason why kitesurfing harnesses have them though.. The forces projected on your spine while jumping, doing kiteloops etc shouldn't underestimated and are on another level then those while casually lifting weights.
(its hard for me to explain the difference of force because of the language barrier)