I think Serge's answer was good.
I wasn't attacking anything myself, but I have had Protestants ask me that very question, and I never had a good answer for them.
I must also admit that the idea of calling a mere sinful mortal "Your All Holiness" bugs me a little bit. That probably has as much to do with my background as an American as it does with the loftiness of the title itself.
Armed with Serge's explanation, such titles won't trouble me (too much).