Personally I love the silence, but it would be fun to have fake sounds on occasions. It would be great when I give demo rides to friends/coworkers. Start with it on, then 1/2 through kill the sound and bask in the silence of the future.
Every person I've given a ride to has been extremely impressed with the Model S, but at least 1/3 of them comment on missing the engine sounds when accelerating. Adding decent fake engine noise inside the car is trivial (a great job for an intern programmer) and I'm positive it would add a few more sales to old school car people who are not quite ready to go silent in a $100k+ performance car just yet..