Some boost libraries are header-only, some are not, and for various reasons etc.
Is there a specific reason/design decision why Boost.ProgramOptions is not header-only?
I'm wondering because it claims to be a "small" library in its the documentation and I don't see any system-related reason (like threads or asio).
Program Options claims to be small, but it turns out to be the second largest library we were building, after Regex. (It is bigger than boost Filesystem and Thread libraries.) I believe that you should be glad they're building a library for it instead of choking your project with a ton of included headers. Perhaps the author thought it would be small when he started and forgot to change the comment when it continued to grow and add features.