High compression engines need high octane gas so they don't knock/ping. Sure, it won't cause damage with today's sophisticated engine management computers, but like has been said earlier it will reduce your power by retarding the engine timing so it doesn't damage anything. Of course, computers can fail, or not go fast enough to prevent the knock.
I used to drive a '65 turbocharged Chevrolet Corvair. Without 91, that thing would ping like no other. Even with 91 when it got too hot and I was pulling hard up a hill...ping ping ping.
And, here on the west coast we can't get anything higher than 91 anyways.