Cuban is correct about how winglet work, the other posts are not. They are designed to be specifically optimized for the cruise configuration. On the MD-11 they reduce fuel consuption 3-6%. Not sure on the 747-400, but I doubt very much they are just marketing.
Although a super-critical wing does not need them as much, they still would help for long haul. They aren't that useful for short haul because you have to choose one flight regime for them to work, and they will actually add drag in other regimes (unless you designed adjustable winglets!).
Consequently, aircraft that are designed to do well in short haul markets have to find other solutions. The 777 used longer wings as well as super-critical airfoil shapes.
Historical footnote is that the MD-11 was originally conceived to have super-critical wings also, but MD chose not to allocate the funding that would have been required. That turned out to be a good thing for the 777, because an MD-11 with super-critical wings would likely burn less fuel for a given sector than a 777 and not be ETOPS restricted. Couple that with the higher payload capability of the MD-11 and we might be in a different world today.