Yes, it is. Ideally, package names should be globally unique, to avoid naming collisions. Using the default package breaks this convention. It's also impossible to import a class from the default package.
Why do unnamed packages exist at all, if it's such a bad idea? From the JLS §7.4.2:
Unnamed packages are provided by the Java platform principally for convenience when developing small or temporary applications or when just beginning development.
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