To reinforce the feedback angle, I think it's important to point out that the weeds starve out the opportunity for genuine content to grow into a valuable audience. Even when there's real content that wants to compete, the risk is the upfront investment required to peek out above all the crap exceeds the profit from serving that niche.
And, in those cases where the content mills do have a few morsels of useful information, they've usually just pushed other less professionally/cynically optimized sources for the same info down off the first page of results.
That's why sometimes I think sites that do manual culling like Quora are greatly valuable. Sure when they have sizeable information, peole will try to spam it too but that spam will soon get cleaned up. Just like wikipedia.
