What reason would a library have to not use a controlled vocabulary if possible? In other words, while LCSH, MeSH and others don’t have all the terms a cataloger might need, it’s hard to see why someone would choose *never* to use controlled vocabularies. If we allowed every library that wanted to put its own “folksonomy” on authority records to do so, I would think the records could get pretty messy.
When you say that “If it appears only in 100 $c, there'd be no easy way to determine what facet category the term belongs to,” are you talking about a computer program determining that? If adding professor to a name makes it unique and 374 College teachers $2 lcsh is also on the record, it’s hard to see what more information one would need. But maybe I’m not understanding what “determining what facet” refers to.