The beauty of "Store As" is that it allows the saving of an edited Rig while simultaneously requiring (forcing) user to assign a new Rig name...all in one action.
you can do the same with duplicate. hit the button button, change the name, store rig. that's not really different. "Save As" would require an extra dialog for the name.
by the way: if you want to "save as" a rig from a rig pack, just drag it somewhere else (where you can save things), change the name, hit "save".