Currently Acoustic Feedback works only one note at a time when using multiple note selection, and selected note is played back only on mouse up. Also, when selecting all notes of a chord in Key Editor, and moving them with up/down keys or mouse, only one note is played.
Polyphonic Acoustic Feedback is only possible when holding Ctrl and clicking a note, at which point it plays all MIDI notes at that time, or in Chord Editing when inserting chords from list.
It would be useful if Acoustic Feedback would play all selected notes when selection rectangle hits them, and when moving selected notes with keys or mouse.
One particular usage for this is “strumming” chords in Key Editor with a stylus when using Microsoft Surface or Wacom tablets or other tablet/touch interfaces. It is useful to quickly to hear chords and test voicings, especially when away from keyboard.
This kind of function can be tested with version 9 of Ableton Live - selecting notes in piano roll will play them. I understand this is also related to what has been called MIDI scrubbing.