In a job interview two weeks ago, I was asked a question about the potential for malicious tagging if libraries opened up their catalogs and allowed users to tag books as they pleased. At the time, I answered by saying that most tagging systems handled malicious tags by ignoring them: if only one person tags an item with a malicious tag in a system like LibraryThing that has thousands of taggers, then that tag will sink to the bottom and the more common (and therefore more useful) tags will rise to the top. But after chewing on the question for awhile, I think I want to take that answer back.
Oh, it's a perfectly accurate answer, don't get me wrong, but I want to contest the very premise of the question. Why don't we as librarians trust our users to act responsibly in the library catalog? Consider all of the trust that we already put in our patrons. We let them wander around largely unsupervised in buildings containing millions of dollars worth of books that people have spent hundreds of hours painstakingly shelving in the proper order. We let them check out hundreds of dollars worth of books and DVDs at a time and take those books out into the world, where we have no control whatsoever over what they do with those items. And, really, how often do our users do anything truly catastrophic when given this trust? Yes, there are accidents sometimes, and coffee gets spilled on books or dogs them chew up, but I'm talking about really large-scale, intentional attemps to be destructive or malicious. Have you ever had a group of users, say, decide to re-shelve a section of books by color? (If I was a bored freshman looking for some sort of mildly amusing prank to pull, that's the kind of thing I'd consider. Think how pretty a rainbow of books would be!) How often do bored students pull the keys off the keyboards on the library computers and put them back on in alphabetical order? (Personally, I don't understand the amusement value of that one, but it was popular with kids in my computer programming classes in high school.) Often enough that you would call it a problem and consider taking some sort of step to cut down on it? If not, then why do you think your users would be any more likely to vandalize the catalog than they are to vandalize your physical facilities?