Application Package Interfaces are useless in Peoplesoft

Let's say I want to mock up a person interface for a couple of implementations.
This is a very basic example:

interface Person
method getPerson() returns string;

end-interface;

Ok now let's move down to the implementation

Class MyPerson implements Person

end-class;

method getPerson
return "here is your person";
end-method;

This returns an "undeclared method" error in peopletools.
Peopletools does accept this, however:

Class MyPerson implements Person
method getPerson() returns string;
end-class;

method getPerson
return "here is your person";
end-method;

The method still needs to be declared...so remind me again why I should go to the trouble of declaring an interface when it saves no time during the implementation.