What is the use case for sharing a term between multiple taxonomies?

The helpful folks on WordPress IRC pointed me to this article, which answered my question:

https://make.wordpress.org/core/2015/02/16/taxonomy-term-splitting-in-4-2-a-developer-guide/

In WordPress 4.2, shared taxonomy terms – those items in the wp_terms table that are shared between multiple taxonomies – will be split into separate terms when one of the shared terms is updated. This change (31418) fixes one of WordPress’s most irksome bugs, and is a critical step in our ongoing taxonomy roadmap.

Furthermore:

The vast majority of terms are not shared between taxonomies; shared terms themselves are an odd edge case.

So basically, this use-case has not been supported for a long time, and was rare to begin with.

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