Here is how I understand winglets. Winglet thrust, in itself, is quite small. In fact, the "forward" thrust contemporary winglets generate by means of vortex interaction barely offset the induced and parasite drag they create themselves. So no increse in "overall" thrust like an engine will provide, so much as a net reduction of self induced drag (pretty clever though).
However, taking thrust aside, the lift and drag they do create as a supplement to the wing is clearly positive and beneficial
