But I do not understand what the point in "Find In Project Files" (a menu item added by Project Organizer!) which simply can't search in project files, as they defined by Project Organizer.
Well, it searches in the project directory where, you know, the project files are...
It has no value since it is just an alias for "Find in Files".
Except that it searches in the project directory instead of the directory of the current file.