Earlier quoted context omitted.
I don't have any Debian machine that doesn't end up with build-essential and dozens of header packages installed; it just comes up too often. Adding the header files directly to the packages saves time and frustration, and probably even saves space since a lot these packages are 50% overhead (dpkg meta files, db entries, etc.) by size.
I refuse to believe that these meta files and database entries are occupying anywhere near a relevant amount of space.
Relatively.
Absolutely, it's negligible - because the space consumption of header files is also negligible. It's such a bizarre design choice to split out a single 30 line .h file into a separate package.