I think both master and parent are needed, for the likely rare case when you don't want a dialog attached to any window, but it needs a Tk window handle to build the dialog from. 

Ran across that in SearchEngine, which has a root window just for the purpose of creating dialogs, but the search engine doesn't know what search dialog is invoking it (a bad example, as in this case, the dialogs should be the ones display the error, not the search engine).

Anyway, have attached masterparent.patch for default branch which updates those message and file dialogs that make sense to use parent instead of master.