With all the fuss about tags recently, Clay Shirky has a rough-and-ready but very spiky post on the 'Many-To-Many' blog which nicely summarizes why tags are important.

The reason ontology has been even a moderately good idea for the last few hundred years is that the physical fact of books forces you to predict the future. You have to put a book somewhere when you get it, and as you get more books, you can neither reshelve constantly, nor buy enough copies of any given book to file it on all dimensions you might want to search for it on later.

Ontology is a good way to organize objects, in other words, but it is a terrible way to organize ideas...