Give the users what they really want and need, instead of what you think they want and need. Of course the trick is having the vision to figure out they want since focus groups and user testing can only go so far! This is one of the many reasons that designing software and web apps that people want and need and actually use is really difficult.
This is something the 'well-designed metadata' crowd has never understood - just because it's better to have well-designed metadata along one axis does not mean that it is better along all axes, and the axis of cost, in particular, will trump any other advantage as it grows larger. And the cost of tagging large systems rigorously is crippling, so fantasies of using controlled metadata in environments like Flickr are really fantasies of users suddenly deciding to become disciples of information architecture.
This is exactly, eerily, as stupid as graphic designers thinking in the late 90s that all users would want professional but personalized designs for their websites, a fallacy I was calling "Self-actualization by font." Then the weblog came along and showed us that most design questions agonized over by the pros are moot for most users.
Any comparison of the advantages of folksonomies vs. other, more rigorous forms of categorization that doesn't consider the cost to create, maintain, use and enforce the added rigor will miss the actual factors affecting the spread of folksonomies. Where the internet is concerned, betting against ease of use, conceptual simplicity, and maximal user participation, has always been a bad idea.