Some of the uncurated tags are indeed very helpful, like, say, a 'magic made them do it' tag. I tend to use them as information about a story I'm about to read. I'm less likely to use them to search, however, although I understand that other people might. But AO3 *has* seen a lot of Tumblr-style tagging, the purpose of which I can't quite fathom in a fic archive. I don't know the first thing about code or what slows AO3 down, but if the freeform tags are a problem then I wouldn't mind losing the search by freeform tags function.