It is so easy to separate spammy link/blog farms from real websites that the very idea the disavow tool is needed by Google is funny. I looked at one that was considered to be on of these a couple of year back, and there was not a single blog post that I could find that had any value. It's very similar to new members joining this forum, posting a handful of replies just so they can put their link in. I get "contributors" every day on my blog, and it takes me no more than 3 seconds to recognise them as link-spammers. The posts are generic, often off topic and then somewhere along the line is a disguised link to a poor content website.
The idea behind links being a good indicator was correct, but it was too easy to abuse and the majority of links are artificial. Google has started to lower their influence, and I am pretty sure it won't be too long before all sites with spam links are penalised. The disavow tool gives us the option of undoing our sins so that after x amount of time there is no excuse to have "fake" links.