Let's say this:
I have a website, I'm reviewing electronic products, one of the product is very expensive and not useful. I want to link to that product with a nofollow link, as I don't want to give reputation to that site.
If google gives value to nofollow links, how google will know if I'm saying that this product is good and should have better rankings or this product is bad and should have worse rankings?
By context, I don't thinks so. If google detects bad words and relate them with the link they are not doing a great job. As I can say: "You should buy galaxy s instead of an iphone because galaxy s is not that bad, ugly and expensive product" How google knows if "ugly" and "expensive" is related to galaxy or iphone.
And a video with Matt Cuts who explain how wikipedia nofollow links are totally useless
http://www.youtube.com/watch?v=x4UJS-LFRTU
As you can see if the link is nofollow is zero for search engine rankings, even if it comes from wikipedia.
That's why I always try to buy and offer in my SEO services 100% do-follow links.
