Well, most of the people try to beat the algorithm and get backlinks from quality sites. What they call quality sites are websites with high
PR , but often you can get a better
PR by getting a backlink from a site that is more relevant to it's own topic even if the
PR of that site is less than the so called quality site. Moreover, in an attempt to optimize the websites , people are perhaps losing the original and simple way of rising on search engines- Get great content.