From memory the MEL permits removal of winglet(s) off the B744 for dispatch with a minimal fuel penalty. And the marketing of the component has always been that it stops spanwise flow and spillage of air from the airfoil to reduce wing tip vortices, but perhaps the biggest driver for thier use is keeping up with the competitor (airbus) and that their use is more for cosmetic rather than truely performance reasons - ie manufacturer seen to be updating a design?
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Its life Jim, but not as we know it!!