Not going to get too deep here, someone else might have the time for that...
Short answer is that it doesn't need them. They have a newer wing that took advantage of the latest from NASA, etc., it also has more span that makes a lot of difference. They are talking about adding them for some future versions, I have heard. The MD-11 should have had a new wing but MD couldn't afford to build the new plant it would have taken, so they just added winglets to the DC-10-30 wing. Unfortunate. They had also at one time considered a twin engine version, but old man McDonnell would have none of it, they say.
For some insight into the issue (besides the book rec I gave of Flightwise in another thread), check out
http://aero.stanford.edu/aa241/AircraftDesign.html