A yes-vote for tagging.
In public libraries, we use the dewey decimal system to keep books in some sort of order and to keep like items near each other, and for the most part it works out quite well. Here at home, I don't keep my books arranged by dewey decimal. It's a shelving-by-feel system--recently read books are together, novels and poetry mingle together, art books are all together, (mostly because they are all large and only fit on a few shelves,) my Complete Peanuts books are prominently displayed because they look so nice, and so on and so on. It's a system that works in this household, and this house only. If I went into a public library and rearanged the books according to my system, it would cause chaos. No one would ever be able to find a book again.
But in an online catalog, there is no limitation by space. A collection can be arranged and rearanged by anyone at anytime and nothing will be lost. Tagging lets me and anyone else rearange a collection in a system that fits me. And because tags are public, if others are using similar terms to tag the same items, eventually new sorts of orders will develop.
I see no reason why libraries shouldn't take advantage of the power of public tagging. It will not ruin the collection; it will allow patrons to increase the usability and relevancy of the collection for themselves. It's not a substitution for traditional classification, it's just something additional that only adds value.
