Apple, the supposed forefront in GUI design, would strongly disagree with you there (have you read the Human Interface Guidelines).
Once I've read 'some' guidelines, but I believe it was on some Linux website. In any case, there's only 1 person who decides what interface a private tool should have: me, myself, and I. Not Apple, not Microsoft, nobody except me. And if I want to use an MDI then what are they going to do about it?
Here's perhaps something to think about: If the whole world agrees on the interface of a piano, does that make a violin builder stubborn then? And before you answer: "a violin has more playing functionality than a piano", I could answer: "dedicated/special controls could offer more functionality than the standard ones".
With that in mind, what about my spinner (see some recent debate about it)? It does offer a lot more functionality than standard OS spinners (which btw are not natively present in BMaxGUI), yet it's a 'dreaded canvas object', probably not designed like Microsoft and Apple would design it. And yet it's so useful and feature-rich.