Bidirectional MIDI enables the following:
- Automatic display of effect and looper states (for example, if you turn off an effect block using the Axe-Fx buttons or Axe-Edit, the corresponding button on the PBC will turn off)
- Display of the Axe-Fx tuner on the PBC display
- One or more buttons can flash at the Axe-Fx's current tempo
- Downloading of preset and scene names
If you just use one cable, you'll be able to control everything on the Axe-Fx, you just won't be able to get those status updates.