Currently over USB we have 3 midi ports
USB MIDI 1, USB MIDI 2 and USB MIDI CTRL
so 2 + 1 special purpose.
this was done since the E1 has 2 physical midi din, so midi 1 and midi 2 map directly to these.
which makes things very simple/clearn
however, USB midi (both host and device) support up to 16 midi ports.
the main question is why separate ports, rather than just using midi channels - after all we have 2 x 16 channels.
this is valid, there are two reason
i) fixed midi channels - some synths/devices fix which midi channels they can use, often to midi channel 1 - using separate ports, gets around this issue
ii) mulit timbral /mpe synths use many channels quick
given we are limited to 16 ‘devices’ on the E1 this is not a big problem except if we are using midi thru
so mainly this is an issue if you use the E1 as some kind of hub.
anyway, I think it could be useful to extend, perhaps to 4 + 1 ctrl.
(this is a bit arbitary, i could even see 6+1 useful, is some limited scenarios)
these additional (ports > 2 ) would be usable only by the USB midi.
I see 3 scenarios of use
a) param ctrl to specific devices
these days many synths have usb device ports, that E1 can host.
so we could output to these directly whilst leaving midi din1/2 independtly
(this tends to only use ONE usb midi port)
b) param ctrl to usbhost
if we are connect E1 to a DAW, we might want to send to different VSTs.
again, it would be useful to have these independent of midi din1/din2 which might have separate instruments attached.
multiple usb mid ports might be useful here…!?
c) USB host → USB thru
this ones a bit more long term, as it really comes into full use with a few more features that have already been requested/discussed separately. (midi routing)
In a smaller setup the E1 can act as a small midi hub.
in this scenerio it would be useful to be able to do
USB MIDI 1 → DIN 1
USB MIDI 2 → DIN 2
USB MIDI 3 → USB DEV port 1 (attached midi device 1)
also if the E1 support a usb hub on the usb device, this could potentially be extended further)
USB MIDI 4 → USB DEV port 2 (attached midi device 2)
etc.
a nice solid example of this would be using the Blokas MidiHub (there are cheaper forms of this, but I know this one 
)
this hub, support multiple usb midi ports, and has 4 mini din in and out.
what this means is combined with the E1 we could do
E1 <-> DIN 1, DIN 2
E1 <-> usb device <-> MIDIHUB <-> DIN1-4
this would give 6 midi din i/o (2 from E1, 4 from midihub)
(hence my 6+1 suggestion at one point )