MAX,
If winglets are optimized for low speed flight they will produce the most benefits at that time. However, if they are set at an angle to gain the most forward lift vector ("thrust") at normal cruise, then at low speeds the angle will likely be such that the winglets have a neutral contribution, possibly worse.
Rather than debate endlessly here, I point you to page 339 of Flightwise, Principles of Aircraft Flight by Chris Carpenter (available at
http://www.amazon.co.uk but not on the U.S. site for some reason). Chris is head of Aerodynamics at the RAF College, Cranwell.
It is also discussed in "The Illustrated Guide to Aerodynamics" by H.C. Smith, 1992 Tab Books, ISBN 0-8306-3901-2, pages 286-288.
Also, to those who make the very silly assertions that Boeing adds winglets for marketing purposes, I ask you one question: If that were true, why are they NOT on the B-777? Surely they wouldn't want to give the A-330 the advantage, would they?
The answer is as I have previously stated. Expect to see them on some future "long range only" 777 if there is demand for such a beast.
The 747-400 gains a 2.5% drag reduction on average with winglets.