When javadoc documents classes in the unnamed package, it doesn't generate
a package-summary.html page, but the "Package" item (with no link)
appears in the navigation bar. This means to view a list of the classes,
you either need to use frames (which some people don't use) or use
the Tree view. It just seems more natural to have a page for the
classes in the unnamed package.
It is also possible to generate and display the overview page when
documenting clases in the unnamed package, but the unnamed
package doesn't appear in the package list.
Also, when mixing classes from named and unnamed packages, it seems
unbalanced, as the Package navigation bar item is not available
when a class in an unnamed package is displayed.