I agree that quality content is important. I currently pull in $300 to $500 each month through AdSense and do struggle to come up with quality content.
However, I do know someone who pulls in several thousand dollars each month from AdSense by focusing on mediocre and crappy articles with unique content, plus a heavy focus on SEO. His theory is that you need to focus primarily on search engines to drive traffic. Once users find the site and determine that the content is crap, they are highly likely to click on an AdSense ad to find better information.
I'm not advocating this approach, but I thought I would toss it in there. I can write a few good articles per day, while he can post dozens of crappy articles. His sites drive much more traffic than mine.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin