In many cases, a developer may want to set up a taxonomy, which each term can only be used once per post, and needs to be enforced on the front end to the users.
Some user cases:
- allow for custom post stati for a custom post type to be managed in the backend
- allow for using a taxonomy terms for a single picklist, this could be used for many things - a lead status field for a CRM, menu item categories for a menu item custom post type.
- applying a single job title or company taxonomy to a user account
- allows the use of categories or a custom post type to be better used in a creating the permalink. A post with multiple categories will pick the first one if categories are used in a permalink - but with a singular taxonomy, a new taxonomy could be set in the permalink structure, categories could still be used for related posts, but the new taxonomy can be dedicated to creating the url and menu verticals.

An additional argument could be made to allow saving the term name as text in the the post meta field, instead of linking the post to the taxonomy.

is_singular would:
- set the taxonomy metabox to a dropdown (or radio?), instead of checkboxes
- force wp_set_object_terms $append to false (not overridable)