The battle inherent in the system
The SO systems of ownership and collaboration are perpetually at war with one another.
For many questions, the best answer would be a combination of multiple answers by several different authors, but is never realized as each author holds on jealously to his own words, either due to a desire for the reputation that can be gained, or simply pride. In some cases, this can get ugly, when a late answer is posted to address errors common to existing answer(s) and then attacked by insecure competitors.
This is regrettable. But, it's a trade-off: this system encourages a competition among authors that often produces multiple answers where each one is better than a single collaborative answer. As authors strive to differentiate their answers, they add value that would otherwise have been excluded.
The single most important Stack Overflow feature
That's not to say we should just throw up our hands and accept this state of affairs as "good enough". When I first read about Stack Overflow, that one aspect described in your FAQ quote immediately caught my eye: trusted users can edit other users' posts. Not just "Community Wiki" posts - any post. A top-rated answer, or a down-voted question are both fair game for any editor with the time and motivation to improve it.
Note that, unlike asking or answering questions, the motivations for editing are almost entirely intrinsic: there's no reputation to be gained, only one badge, and precious little in the way of recognition. Editors must be be satisfied with seeing a question or answer improved. IMHO, that's not a bad motivation! Once a user has been on the site long enough to gain full editing abilities, a whole new way of interacting with Q&A is opened to them: no longer must they struggle to get their "errata" answers or comments noticed, or cringe while responding to a poorly-formatted question: they can see a problem and fix it.
Not that everyone likes this idea. Believe it or not, some of the most knowledgeable and prolific authors on SO are among the most reluctant to edit other user's posts and jealously protective of their own posts. IMHO, this attitude is actively harmful to Stack Overflow - not only does it run the risk of discouraging editors who find themselves embroiled in an unexpected argument with a protective author, it sets a bad example for new users, who look to these highly-visible users for cues in how the site should be used.
I don't think we need more motivation. SO may not fulfill some utopian idea of what a wiki system should be like, but it was never intended to be a straight wiki. The system, freakish monster that it is, actually works for a good many people looking for solutions to their problems. And as many of us can attest, the joy of seeing a poor question turned into a good one... or a good answer turned into a great one... can be more than enough to keep us going. If anything needs to change, it's the attitudes of those who would callously discourage new editors.
Dustin inline response:
I agree with your whole post. The anti community ownership culture IMO arises out of the incentive system. The culture will not shift unless the incentives shift.