When an organization is already having good taxonomy in place why people use Folksonomy instead?
What is actual need Folksonomy or social tags?
Taxonomy is structured and controlled. Folksonomy (Enterprise Keywords) is unstructured and open. A file may be classified with the business taxonomy. The individual user may still want to apply keyword tags that have a special meaning to that individual user and help them find the content they need.
There are also techniques and tools to review the folksonomy terms on a regular basis to determine if some of them should be moved to the taxonomy instead.
Things change. Things evolve. The taxonomy that was created two years ago may no longer suit the business needs perfectly. Folksonomy is one area where required changes to the taxonomy can be discovered and be dealt with.