Sometimes the right defaults depend on context unavailable to the low level library author. But irrespective of who provides the higher level API, the bottom line is that there needs to be an API that's most flexible when the flexibility is warranted. This is really in reference to Colin's "nothing more to add" remark.
If there is some other context that is unavailable to low the level author, then that default should be provided by whatever higher level is providing said context. C++'s namespaces provide a super convenient mechanism to make that work smoothly.
