Ten academic people can advocate why MDI is bad, but if there is a single artist who really has a purpose for MDI, does that still make MDI bad?
Are you really suggesting I spend my free-time coding patches for that one in ten person who thinks that MDI is what they want? Surely MaxGUI needs attention in places where most people will find it useful.
Whether MDI is good or bad, I think a GUI should support it and hand over the possible compatibility risk to the creator,
Apple, the supposed forefront in GUI design, would strongly disagree with you there (have you read the Human Interface Guidelines). Even Microsoft have become a lot more strict recently with letting the programmer hack away of user interface standardization. But that's a completely different matter...