I would argue that people with the capability to recognize an affiliate link should also have the intelligence/judgment necessary to determine whether a review is unbiased or not. In effect, I think a "sponsored link" disclaimer is pointless. Personally, when I see a such a disclaimer, I practically assume that the article
will be slanted. I would rather make that judgment myself.
Furthermore, as I pointed out it's not only affiliate links that can slant a review. I'm taking any review I read with a grain of salt, and my estimation of the author's or publication's integrity is a large factor in how much faith I have in a review. This is why it is such a big deal when stories come out like the TechCrunch author who demanded merchandise in exchange for reviews.