I can provide evidence that this is a horrible idea.
Try typing a phone number into google.
Go ahead, try it.
Google has improved in some regards. I recall having to look through 3-4 pages to find a decent result in some queries a few years ago. They're usually on the first or second page now, but there are still these worthless results that end up in the 1-3 spot. That's enough evidence for me that they need expert intervention, not algorithmic guessing.
besides google reader, most of the cuts have been good or just not important. I always felt like this feature was a reaction to blekko search engine, and not very interesting. curiously i found myself not wanting to block crappy sites like ehow just cuz they might have a good page and would rather just trust google.