Adding a winglet is more than just attaching it since it changes the load on the entire wing. And then you have to consider the costs associated with the additional weights, cost of installation, etc.

Airbus had previously said that gains achieved did not warrant the increased costs and weights. Also, each winglet has to be specifically designed for that wing to achieve maximum benefit.

I talked to the guys at AP a while back and they said a lot of stuff being attached to wings are 'pretend-lets', not winglets.

They are looking at even more advanced designs such as the spiroid.