why do you think MDI is a failed experiment?

OK, that was a bit of an inflamatory stance :-). Let me explain ...

I almost always have multiple windows open. I prefer to Alt-Tab between my most recently used windows and almost never click on a task bar icon/button. The MDI interface breaks that functionality. Instead of thinking "I'll hit Alt-Tab 2 or 3 times until I get to what I want" I have to think "which application window is the document in?", Alt-Tab to that and then think "which client window is it in? and which hot key did the application vendor use for cylcing between open documents?" To make matters worse there is no standard for the hot keys and some applications always cycle through documents rather than using a most recently used stack.

I was under the impression that MDI was a Microsoft 'innovation' - although I'd be happy to be proved wrong. My first encounter with MDI was certainly with MS Word, Excel et al. The reason I say it was a failed experiment is that recent versions of Microsoft Office apps no longer default to MDI and in fact I don't think they even support it any more.

When Opera first came out, my first impression was oh dear thay've used that nasty MDI interface - and they seem to consider it a feature. Surely if MDI was such a good idea, Microsoft as it's principal proponent would have used it in their own browser.

It is certainly true that opening lots of top-level windows chews up icon space on the task bar. Perhaps that is a problem with the task bar rather than the windows - I don't know I don't use it.