While the idea of making .pkgmeta externals grab the latest tag, instead of the latest revision, by default is a good one, I'm slightly puzzled as to why addon authors are tagging versions of their addons with broken libraries in the first place. If you're developing an addon that uses libraries, and you set up your .pkgmeta to grab the latest revisions of those libraries, why are you not keeping your local copies of those libraries up to date?
it just seems that using tags:latest might not be the best way to go (its pretty much the same as using trunk - tags dont always mean release worthy code, theyre just a certain point on the codestream) but using tags:xxxxx would be more robust as at least then you'll know what youre going to get.
The way the CurseForge packager is set up, authors shouldn't be using tags to mark random points in development, as the packager automatically marks Beta and Release versions based on tags.
Edit: Also, I remembered the other (bigger) reason why subfolders are better: if the library doesn't have a subfolder, then your addon ends up including a bunch of copies of LibStub and CallbackHandler-1.0. :|