Sometimes taxonomy is a tool and sometimes it's a cage:

The task of the taxonomist or information architect is not to provide absolute consistency and standardization, maximum tidiness, and complete information efficiency. Optimizing efficiency in a complex system, as Jacobs noted in regard to cities, destroys the resilience of that system and its capacity to adapt to new circumstances. So the task of the taxonomist or information architect is not to optimize efficiency, but to optimize effectiveness, and that always means sub-optimal efficiency. Consistency and standardization must be sufficient for effectiveness and the meeting or your goals and no more than sufficient.

To remain resilient and adaptive, a knowledge environment must always also be hospitable to alternate mechanisms of knowledge organization, access and use – which to a degree will compete for attention with the formally privileged mechanisms such as taxonomies.

Folksonomies are a case in point. It is not especially healthy just to try to bend folksonomies to the needs of taxonomies as vocabulary harvesting devices as several organizations have done, and leave it at that. If the conditions are right to support healthy folksonomies, then the organization will get far greater value by actively exploiting their potential for providing rich serendipity as well. [I’ll talk about folksonomies and rich serendipity in another post]. They can by all means be used to harvest vocabularies, but this is just icing on the cake, not the substance of the cake.

Also:

To see an example of this plethora of competition between knowledge organization devices we need go no farther than online bookseller Amazon. Look at any Amazon page for a given book, and you will find a taxonomy (represented by formal subject categories), user-contributed tags, links to other books bought by other people who bought this book, booklists compiled by users on related topics, suggestions for other books based on a complex algorithm combining your past behaviours and those of others, and so on. All of these mechanisms for purposefully finding – or serendipitously discovering – books, co-exist, and compete. You can bet that Amazon watches the intensity of use of each of these mechanisms, and as any single instrument gets used more or less intensively, Amazon will adjust its investment in supporting it accordingly. As Amazon has grown, so has the number of ways of locating and suggesting books. Taxonomies form only part of this complex web, and rightly so.