The fundamental tension in tagging systems, to me, is whether tagging is a feature the software offers to the user or a task the user performs to assist the sof

The fundamental tension in tagging systems, to me, is whether tagging is a feature the software offers to the user or a task the user performs to assist the software. In the first case, you want freewheeling and tolerate ontological inconsistencies because you want to offer flexibility to users and will capture hard to quantify emergent benefits (some made up examples: "try the tag user233-favorite, I keep discovering awesome articles!", "the physicist-needed tag has highlighted a lot of misinformation surrounding quantum physics and relativity"). People use it to the extent it is useful. The other way, with formal semantics, governance (which you made some very wise points about), etc allows the software to reply to queries like "19th-century + Missouri + humorists" in a performant and authoritative way. It's not really a feature so much as it is a way to enable other features.