C++ FAQ Celebrating Twenty-One Years of the C++ FAQ!!!
(Click here for a personal note from Marshall Cline.)
Section 22:
[22.1] What's the big deal of separating interface from implementation?

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 time of more expensive people.

Since interfaces are so valuable, they should be protected from being tarnished by data structures and other implementation artifacts. Thus you should separate interface from implementation.