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LaTeX3 provides tmpa and tmpb scratch variables for many different types. As documented, they may be overwritten by other non-kernel code and so should only be used for short-term storage.
What is exactly short-term storage ?
I would say "a group with only kernel, private code, and no third party code whether explicit or not". This seems a serious limitation for the usage of such variables.
The scratch variables are safe if you have no third-party code between setting and usage. As the core of expl3 does not use these variables at all, you can have basic functions intervening without risk.
Personally, I tend to think of the general scratch variables as mainly aimed at use in ad hoc code in documents. They are also useful for genuinely one-off operations during package loading. Anything that is used repeatedly in a package is likely better using a private scratch variable.
