3D is great and certainly improved, but...
I hardly think that gaming and entertainment should take priority over creativity and productivity.
I remember (this was about 10 years ago) when MorphOS didnt have 3D acceleration for Radeon cards there was somebody yelling how MorphOS could be taken seriously without 3D... to some user other things are more important that others.
The last I looked, USB, MIDI, Sound Sampling and so on are pretty basic needs. You don't have to have Windows or OS X to take advantage of those features, you can use an old Amiga, and Atari ST, a C64, Atari 8-bit, even DOS for most if not all of these needs, so this is not just something that has recently been added to OS' and hardware, and it certainly would not hurt for MorphOS to FULLY support they hardware that they began support for back in MorphOS version 2.7.
Neither my Amiga 500 or Commodore 64 support sound sampling or MIDI...

as far as USB isochronous transfer support is concerned it is not something that very often. Anyway, I have no idea if isochronous transfer will be supported. It would be nice to have so Poseidon support would be complete but it is not my job.
Anyway, at least you have raised the issue. If nobody ever ask then nobody care...
EditFor Maestrix emulation have you tried this:
http://aminet.net/package/util/libs/TheMaestrixI dont have OctaMED SS so I cant try it out (and you probably already tried it?). If it doesnt work I dont know if there is something else blocking OctaMED SS but again... I never had that program.
To use MIDI you would need a computer with old style serial port (Pegasos 1/2 or Efika ?), it certainly is supported in MorphOS but to my understanding MIDI via USB would require isochronous support... so it comes back to USB then.