Lots of good ideas/suggestions here.
I can think that there are (at least) two ways of looking at it.
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It can be useful for relationships to have properties themselves. So for example, a relation to an organisation could have a checkbox to indicate that it is ‘current’.
See here:
Relationship properties
although this did not explicitly mention the exact use case being discussed here. -
An entity has a history, and sometimes it’s useful for other items to link to a specific historical version of an entity rather than the current one.
See here:
[APPROVED] Versioning in Rich Text Fields (and entities as a whole, too) - #6 by Chr1sG