Agree separating the interface and implementation is not a great idea. It seems like a decision that's designed to waste time (in copying definitions from one place to another, in compilation errors).

It's half the reason to dislike C/C++'s module... err, text inclusion system. The other half being horrible build systems/times.

The only way it'd be halfway reasonable is if there was very good editor support for syncing the two copies. And even then, what's the point of denormalizing your source code? Like you said, you can always generate docs from source or an interface file from source (if for some reason you were building a closed source module -- which seems very much not the norm these days).