Separating interface from implementation is a key to reusable software. Interfaces are a company's most valuable resources. Designing an interface takes longer than whipping together a concrete class which fulfills that interface. Furthermore interfaces require the resources of more expensive people (for better and worse, most companies separate 'designers' from 'coders'). Since they're so valuable, they should be protected from being tarnished by data structures and other artifacts of the implementation (any data structures you put in a class can never be 'revoked' by a derived class, which is why you want to 'separate' the interface from the implementation).
