Boeing found, with the 747-400, that the comparable increase in efficiency with increased wingspan was greater than by having winglets. This led the company to do some serious research into wingtip devices. Boeing settled upon the raked wingtip design (not the sort used on the 767-400ER, but the 777 and 737NG) as it did not incorporate unnecessary structural strengthening required for winglets (they twist in flight and the wing must be made stronger).
In essence, instead of a winglet, Boeing just increased the wingspan to the weight of what it would have been with a winglet. More performance was generated in this area.
Then Boeing discovered the blended winglet, which does not require the same level of structural strengthening and older types. Boeing, ofcourse, has a patent pending on the 767-400ER raked wingtip, which is the best of both worlds - winglet and increased span, with significantly less weight.