This seems to be changing on sites like HN and Stack Overflow. The only difference I can see is that the moderators intentionally guide the discussion away from trolling. Are there other factors here that I'm missing?
I think the points systems on such sites give a different incentive. Many people help just to help, but others take part primarily for the point gaining (which rapidly becomes addictive) or to gain some sense of achieving more than others. Despite these motivations, if the ultimate result is a good one, I see no harm.
It was one of my ambitions to have a book published, so the primary motivation was achievement. My book has helped thousands of people learn to program, but if teaching were my primary motivation (it was certainly a secondary one), I'd have written and released it for free online (and even then the motivation might have more been fame and recognition than true philanthropy).
As it is, the second edition should be coming along next year, and I'm hoping to have the e-book version freely available for anyone to download. Primary reason? To promote the book and get people to buy the print version. That more people (who might not be able to afford the book) can learn is awesome, but that's a cool secondary goal :)
Sorry if this all sounds a bit dry, but I think it's how people truly think. I have good intentions, but they're usually for me (and my family) first, and then if other people benefit, awesome! This is why it's hard to make decisions based on other people's intentions if you don't consider how people really think. With few exceptions, we're all more selfish than we think (selfish in a good way, often).
Not to mention, writing a book gives you more credibility when you have to tell someone their operating system is an overpriced box of bugs, therefore assisting the ego in defeating the forces of cognitive dissonance that seem motivate people to defend their own positions.
