An obvious benefit to the sound of the ICE motor kicking in is that it will be a reminder to the driver that they are now running at up to five times the transportation cost and are now polluting, importing foreign oil etc.
Just by buying the Volt it means they care about some combination of these causes. The sound will teach them to head home and not wander, combine trips, make shorter runs, always plug in and generally drive more conservatively.
A nice primer for when they go full EV
