Not sure if I agree. I find that coming up with good interfaces is often more of a creative challenge than the implementation. API's are worthy of some sort of

Not sure if I agree. I find that coming up with good interfaces is often more of a creative challenge than the implementation. API's are worthy of some sort of protection since ease of use is a definite competitive advantage. Also, you can easily screw your perf with bad interfaces (especially in C++). E.g., not templatizing, requiring extra allocations, type impedance mismatches, vtable bloat, etc. The interface certainly affects how the machine executes your code.