"The future co-existence of controlled vocabularies and collaborative tagging is predicted, with each appropriate for use within distinct information contexts: formal and informal."
"(...) tagging system is not "controlled" in this sense (...), but I'm wondering whether its web-scale nature can provide some benefit that one would not expect."
C. Wartena. Research and Advanced Technology for Digital Libraries, volume 6273 of Lecture Notes in Computer Science, Springer, Berlin / Heidelberg, (2010)