Wake vortices and winglets
Does the amount of energy being dumped in turbulence off the surface of the wing matter for drag?
An aircraft that weighs 200 tons and has a 50m span, flies at 200 m/s in air with a density of 0.1 Kg/m-cubed needs to deflect the air in its wake at a vertical speed of about 40 m/s to stay up. That requires about 10MW of power. Managing the rotational flow that that descending slab induces compared with the air outside the span is important for reducing that power demand. By comparison, I suspect that having laminar flow over the whole of the wing is not a big deal, unless you're dumping more than 1MW into the turbulent flow. With a 50m span, that would be 20kW per meter of span, which seems to be a lot. It may be true, but I suspect the effects of winglets for sculpting the large-scale vortices behind the aircraft are much more important for drag reduction than changing the boundary layer turbulence properties near the wingtips.