Palgia,
My reason for quoting the CD intake was to illustrate another example of a system whereby energy that would otherwise be lost is recovered and used to increase thrust. Like the winglet, the CD intake does not actually introduce any extra energy into the system, but simply recycles energy that was initially released by the engines.
You are of course correct in saying that the CD intake does not create thrust directly. It actually creates extra drag. But in concert with the engine it increases the total thrust generated. This is different from the winglet, which creates thrust directly.
I am not entirely sure that the use of CD intakes is inevitable, but it certainly increases efficiency. We could for example us some form of inlet lying flush against the side of the aircraft. This would minimise the drag but would not recover the kinetic energy of the air. In effect you would have the required low speed airflow, but at ambient density. The principal problem with this solution is that becuase the intake could not recover the kinetic energy from the airstream, the engine would not benefit from ram effect. The overall result would be a minimisation of the intake drag, but no extra thrust.
Getting back to the winglets, if their only function is to reduce induced drag then the use of tiptanks would have the same effect. If these were area ruled the benefits in high speed cruise would be even greater.
If the objective was simply to restrict the flow of the tip vortex, then a simple end plate aligned directly fore and aft would do the trick quite well. But winglets are not aligned directly fore and aft. Their chord lines are angled slightly outwards. The purpose of this feature is to provide the required angle of attack to generate a forward tilting total reaction, a small part of which is thrust. As MFD stated earlier, by deflecting the vortex flow aft, they create a forward thrust.