You always had to check the individual fields for the zero value. A required field in a proto2 message can be set but also have the default value and pass initialization.
No, you didn't. A required field has a value, period. If it defaults to a particular value, then that's the value it has.
If you had a non-required field, then you marked it 'optional', and checked for the field's existence (or mapped optional fields to a Maybe/Option monad representation, forcing the issue).
In proto2, default values are for optional fields. The value of an optional field would be that value, but there's a separate concern as to whether that field had been set.