I can't imagine too many cases where readline would get pulled into my proprietary code as a transitive dependency. I mentioned psql above: it's not as though I'd have `#include "psql.h"` in my own project.

What's this friction between readline and other licenses in other Free Software code? Unless I'm writing some software with a non-GPL license and want to include GPLed software in my own that I plan to distribute, it doesn't affect me.