WowAce

The .toc file Interface value is used by WoW client to determine whether or not an addon is out of date.  Best practice would be to ensure that libraries in active use, such as this one, are kept up to date alongside the WoW client interface.

In the case of Curse Client or by-hand installations of addons utilizing libraries separate ("disembedded"), we (the user base) are presented with two options:
1) Edit files manually, or
2) Enable loading of out-of-date addons.

This situation appears non-optimal, from both a support and end-user perspective, as well as from the perspective of addon authors that may find they are troubleshooting someone else's out of date addon that is now inadvertently running alongside their own up-to-date addons.

I'm not aware of a reason to avoid updating the TOC Interface value for libraries.  Yet, I see quite a few libraries that appear to be purposely not updated with each Blizzard Interface update.  For this reason, I am hesitant to commit changes to library repositories, otherwise I would do it myself (instead of writing up ridiculously cumbersome bug reports, such as this one).

There is no reason to even install a LibStub standalone, because usually every library bundles LibStub (yes, even in disembed mode). LibStub was never designed to ever be run in stand-alone mode, except for debugging purposes. Its not considered a "library" by itself, its just a loading stub that is supposed to be embeded into libraries.

LibStub is a finished product. We're not going to release new versions just to update the .toc Interface versions, especially considering the above.

I'm not sure why the Curse Client even wants to install LibStub, either itself messes up, or some addon/library set its dependencys wrong. I suggest you bring that up with the CurseClient team.