"MIDI Polyphonic Expression" and is a new MIDI standard created by us, ROLI, Apple, Moog, Haken Audio, Bitwig and others for communicating over MIDI between MPE controllers ... The principal reason for MPE is to get around a limitation of MIDI: Pitch Bend and Control Change messages must apply to all notes on the channel. This prevents polyphonic pitch bends and polyphonic Y-axis control (which uses Control Change messages) over a single MIDI channel. MPE solves this problem by sending each note's messages on a separate MIDI channel, rotating through a defined block of channels. Here's a brief summary of MPE: http://www.rogerlinndesign.com/mpe.html
Today there seem to be work arounds, not that a new midi version would not work better, however.