I’m considering using public sublibraries for publishing .Internal modules.
.Internal modules clutter the package’s module listing in Hackage and make it more confusing to scan. It’s no big deal but I find it annoying (see the module list for text for example).
Also, putting them in a public sublibrary would make cabal build-depends: sections more informative, because it would be evident at a glance if another library depends on the internals or not.