Philosophically, MDI is probably not a great way to do most things. It makes more sense to have the Window mamager manage windows, but unfortunately, we moved away from MDI without really coming up with a solution for providing the advantages of an MDI through flexible WM solution.

Mac OS 9 did this well, but that was because it was effectively single tasking in terms of the user experience.

For Hi-Fi systems, MDI was skeuomorphism too. Actual sound systems had separate boxes that did graphical equaliser function, amplifier function, tape function, CD player function, etc.

When portable CD players were still chunky companies would give them a bit more funk, buttons that followed the curves of the device and such. Similarly, before MP3 players went tiny there was some extra space for a bit more design that some Winamp skins would mimic.