Public writeable fields are threading and encapsulation smells but public read-only fields just say, this data is part of my interface.

well, yes however fields cannot be part of an 'interface' in java. (they can but they'd be static)

I do use package private fields liberally, though. Just not non-static public fields.

Interface as used here means the broader concept of a "a class's interface is its public advertised operations" and not the specific Java thing called an interface.