One of the most interesting lessons of Web2.0 is that even the most open communities need a form of hierarchical governance if they are to continue to serve their members well.

As as any web platform becomes very popular, so it becomes more enticing for people to game it to promote their own interests. If this is allowed to continue then the system can become cluttered and the ordinary user's experience suffers. So, regrettably, it's often necessary for someone to decide what is allowed and then kick out people who aren't playing by the rules.

Here's a talk by Jason Calcanis at Le Web which I think pretty much explains the importance of curatorship of web communities (with thanks to Charl for the link):

It is also important to clearly state the Terms of Use for any community site. We learned this lesson when was criticized for the founder's direct involvement in blocking certain users who were deemed to be spamming the system. Although I believe Neville's actions helped keep the site useful to it's users, not having public guidelines may have made it seem like a personal decision taken against certain users. Muti now has clear Terms of Use, and the community is flourishing.
AuthorDave Duarte