If one class in a package is designed for reuse, all others in the package should be. The moment a package contains one class that is not designed for reuse, its suitability for other purposes is lessened.
Our project needed a custom built app to store and manage documents. Each document was to be indexed by a unique GUID string. Luckily our company already had a library JAR and web application designed for this purpose, and it seemed like we could use it.
However a few critical classes within the package assumed the document ID was a long and did casts internally so the library could not be applied without a major refactor. Worse, some of the classes also had private methods and internal data types that prevented easy extension. No reuse benefits were gained from the library.