In Summit firmware 2.2 we've revised the Arp>MIDI behaviour to meet the following design:
When Arp>MIDI is on, notes played on the Summit keys will be arpeggiated and output as MIDI. MIDI from an external source is not arpeggiated.
When Arp>MIDI is off, notes played on the Summit keys are not arpeggiated to an output as MIDI. MIDI from an external source is arpeggiated.