Yes, I have nothing against using "Back" button for something else than to leave document.
The only thing I want is that if I press it (by mistake), I will not leave the document.
But, for consistency reason, I think that possibility to setup that button to leave or leave with confirmation is the best solution. Everyone will be happy.
You are right that using button for "leave with confirmation" may be considered as waste, but in that case also Menu button is sort of pointless (or Menu button on toolbar) and so on...