Simone, I agree that if you can get a full refund for the lens, it makes sense to go for it, or ask for a replacement lens.
Sometimes Nikon replaces the lens with a new one if it is the most cost effective way to a functional lens. Howeve, for that they would have to admit the lens is defective. If is is easier to get a replacement or refund at the camera store, I would go that route.
Bjorn I completely agree the subtle differences in sharpness may not be real world significant but I tend to try to minimize sources of uncertainty and blur, no matter how small, as in real world shooting the sources of blur tend to add up, eventually leading to something that the eye can see in print.