Filing this under 'entity system' as the original issue #1642062: Add TempStore for persistent, limited-term storage of non-cache data was filed under that too.
There is still a @todo in the class saying that we might want to add these methods if we need them. I strongly believe that we WILL need them. It definitely increases protection against potential security vulnerabilities if we make it convenient for implementations to check for the owner during get/set. Actually, I am currently working on a D7 module which leverages a lightweight backport of the tempstore in several places and can therefore verify that it definitely improves DX if we have those methods.
