I am not an aerodynamics guru, but I had a conversation with one recently. He said these extreme winglets were really a waste of money due to the additional weight and strengthening required in the wing to support the winglet. He stated that it takes very long segments (7 plus hours) to realize much fuel savings from them. The real reason airlines (and leasing companies) keep having them installed is the accountants believe the airplanes will keep their value longer and they will be easier to re-sell or re-lease them later in their life cycle. Thoughts?
edit - I should say he was a self proclaimed aerodynamics expert.
Tell him not to quit his day job, or just be straight forward and tell him he's a moron.