That's my understanding too: at low speeds, the winglet does not add anything (or much). And since it has a nontrivial weight to it, the take-out / climb-out requires more fuel consumption.

I've noticed that WN uses a mix of wingletted and nonwingletted 737's. I've wondered if they purposely assign the wingletted planes to their longer routes. Knowing that there always seems to be logic and purpose behind everything that WN does, it would not surprise me at all.