The problem with that is that USB mass storage works (by specs) by giving direct access to the hardware. So direct that the device's OS (Android for example) needs to stop pretty much all activity to be sure to not touch the hardware at the same time as the computer's OS. That's why you can't do anything else (listen to music, etc) while your phone/player/etc is connected as a usb key.
If you want to fake a filesystem, you'll have to fake the USB hardware controlers below it first. And that's certainly trickyer.
MTP and iTunes allow concurrent access by the host and the guest, and that's arguably the only good thing about these protocols.