Running an engine with 11:1 compression, at altitude above about 4500 feet, I can get by with 89 octane. (less air for the compustion process) As I start to drop in elevation, I must increase the octane as the amount of oxygen increases. At or near seal level, it becomes a chore to keep the car running smoothly. Of course this is a non-computer controlled car, and the SSR programaticly compensates for variations in O2.
My point is, aside from a few minor implications, (burn rates, etc, which are a function of many variables other than octane rating) the higher octane simply prevents pre-detonation. If a vehicle will not pre-detonate with 87 octane, (other factors aside) the higher octane fuels do not serve a purpose, and are indeed a waste of money.
This is a subject that could be argued for days with many variables. I usually run 89 just to be a step above the 87 and to compesate for any possibility that the 87 might not quite be 87.