Keeping packages tied to files really helps me organize myself with respect to testing. It just makes life a whole lot easier if I know that these tests deal with this file, which contains this module, etc. The bigger the file gets, the more likely I am to forget to test something. Smaller files yield smaller test files, which, for me, enhances maintainability.

The same goes for general maintenance. If each package contains a file, I don't need to remember where a particular package is. The file tells me. File modification dates are also meaningful; if the file hasn't been modified, neither has the package within.

Some of this is mirrored by a good revision control system. But what does it hurt to keep things simple?