Because, as they say, self-plagiarism is style, I will quote myself
As for Knols, anyone who thinks Google's initiative is about anything other than the fact that Wikipedia has no Google ads is barking up the wrong tree.
Science Library Pad - Wikipedia death watch? - January 2, 2008
I'm pretty sure I remember Google saying something about "authoritative content blah blah" but the reality is that (like Wikipedia) anyone can create a Knol. The main difference is you are the page admin, so (if you want) you can decide whether to accept or reject any edits that anyone else makes (or deny non-author edits entirely).
And, oh yeah: AdSense ads.
So you can contribute to Wikipedia, a vast and interlinked set of pages with high traffic.
Or toss your Knol out into the wind and hope it is gently lofted to the top of search results.
They're not very keen on you using a Knol to drive traffic to your site
We don’t allow pages that have the primary purpose of redirecting visitors, acting as a bridge page, or driving traffic to another website.
It would seem it does open up yet another front in the spam battle.
In theory with Google Docs you could create docuspam, I guess knolspam is the next frontier.
It seems to me this is mostly about ads, and secondarily about drawing you even further into the web tracking dream: you're always logged into Google, they know every search you make, every email and document, and now they will know your particular areas of expertise, and topics of interest. Seems like lots of benefits for Google...