There are many ups and downs to adding a user generated content section to a site. It has been interesting watching the effects of SEOMoz's user generated content and points systems. The ups:

• users feel they are part of the brand.

• they are more likely to push the brand and link to the site
• points are created free but give some perception of value
• users create free content for the site even when you are not doing so.
• some of their content will rank in search results. today I did a search for search engine marketing and saw Google listing a link for recent blog posts listing this post
• contributors might give you good marketing ideas or help you catch important trends before competitors do

The downs:

• people who spend lots of time contributing tend not to value their time too much AND are hard to profit from (especially in savvy marketplaces that ignore ads).

• having many relationships allows you to be a connector that knows someone for just about any job, but focusing heavily on building community and maintaining the many relationships needed to do so may hold you down on the value chain. A few strong relationships will likely create more value than many weak ones, especially as we run into scale related issues.
• if your site is not authorititative, user generated content may waste your link authority and lead to keyword canibalization
• if your site is authoritative many people will look for ways to leverage your domain or authority
• as you get more authoritative more people will try to exploit it. even friends get aggressive with it, and unless you call people out it gets out of control quickly.
• as you extend your commitments, spending time to police a site, it is harder to change course. I get frustrated when I see spam on the homepage of ThreadWatch, but I guess I can't be surprised people do it, and due to database issues I am uncertain if I will be able to upgrade TW without just archiving the old information and switching to a new CMS.
• some people looking to promote their work may spam or aggressively associate your brand with the articles they wrote. For example, is this comment spam? Or is it good?

If a relationship is affiliate based it is quite easy to police undesirable activity by banning accounts, but if people are adding content to your site and marketing it aggressively in ways that may not bode well with your brand it might be harder to police it, especially as you scale your community. And typically the people that are most likely to give you crap for it are hypocritical with their beliefs.

I think on the whole a community section is a pretty good idea if you tie it into a paid content model, but even when you do that you will still run into scale issues if you provide any type of support for the paid content. I have over 600 emails in my inbox, and recently stopped advertising free consulting with an ebook purchase because I stopped scaling as a person. As your profits scale the opportunity cost of any one revenue channel become more apparent. That is one of the things which has prevented me from putting a forum or community section on this site.