Trypnotik - 4:59 pm on Feb 28, 2012 (gmt 0) [edited by: Trypnotik at 5:18 pm (utc) on Feb 28, 2012]
In regards to link evaluation...
I would speculate that it is directly related to PageRank or how it is passed from site to site. For a long time Google has been telling us that PageRank is a less important factor in how important a link is. So much so that they removed it from their Webmaster Tools interface two and a half years ago.
Google is always tooling with how PageRank is used. An example is in 2009 when Matt Cutts announced that nofollow links had been consuming PageRank, even though the PageRank was not being passed to the linked site. This effectively put the kibosh on simple PageRank sculpting.
Additionally, social sharing is done primarily with nofollow links. Social media is quickly becoming a more significant factor in how sites are ranked and indexed. It's logical to me that a new way of determining a page's authority, and passing that authority between pages and domains, is necessary to accurately determine the true value of a link.
[edited by: Trypnotik at 5:18 pm (utc) on Feb 28, 2012]