The .toc file Interface value is used by WoW client to determine whether or not an addon is out of date.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2) Enable loading of out-of-date addons.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perspective, as well as from the perspective of addon authors that may find they are troubleshooting someone else's out of date addon that is now inadvertently running alongside their own up-to-date addons.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).