Folksonomies vs. Taxonomies

In Folksonomies: A User-Driven Approach to Organizing Content, pretty much Joshua Porter says that taxonomies are great but they're a pain and very expensive to create. On the other hand, though, folksonomies are good because they're user driven. Based on what I got from the article, I wonder why folksonomies would even be stressed. If something that is user driven is better because of the language and cost, then why not focus on taxonomies? I do like, however, how at the end of the article Porter goes on to say that the taxonomies have the "ability to let users do most of the organizational work of the information on a web site, they may yet prove to be a valuable, time-saving way for information architects to keep a handle on the addition of information into an already-burdened architecture." I think that he had a good point. My question, then, is what's the real value in folksonomies if taxonomies already do the job. Maybe it's just me, but I don't get it.