This is probably a question better suited for a wxPython or MSDN newsgroup. What OS are you referring to? What GUI toolkit are you using?
Microsoft's office on Windows has moved to a model where every document has its own toolbar, menubar, and taskbar entry. Windows developers tend to mimic MS Office, so many are also moving to this model. Mac apps have never had MDI. -- http://mail.python.org/mailman/listinfo/python-list