I’m probably overreading into your use of “need”. And the only MPE controller I have is the Linnstrument. So grains of salt on both accounts. But the full-sized Linnsturment can be powered over DC wall wart or USB. And the USB doesn’t have to be special, as far as power goes. It can come right from a phone charger.
For everything else, it’s more than happy to transmit over 5-pin DIN MIDI. That usually peaks at around 8ish channels’-worth of 3–5 CC streams (plus note on/off which are a rounding error. And one channel of more sparse global messages) but I haven’t had anything fall down under the pressure, yet. I mean, I can generate that much traffic with normal MIDI and a Pyramid without being too ambitious.
Certainly it’s way, way more convenient to run everything over a single wire, so I love it when a synth supports a host port. And we’ll soon be at the point where we want to sequence multiple MPE instruments from a single device, and we’ll need way more bandwidth than a single MIDI port allows for then.
But for now, connecting a controller an instrument, it doesn’t feel like a need. Just a nice-to-have.