If you never get on the throttle hard, never even get half way to redline, why the extra expense then? Sure you'll be losing some power, but if your driving style doesn't benefit anything from it what are you gaining?
While there may be truth to your theory, the problem is with pre-ignition. Lets say you fill your tank with 87 octane in your performance car. You step on the gas, engine speeds increase, so does timing. Soon, that highly volatile 87 octane starts going off just a little too early. So the knock sensor detects pre-ignition, the computer tells the ignition system to pull timing to stop the p[re-ignition, and your engine starts to run less efficiently. Now you have an engine that may be running on cheaper fuel, however is now running less efficient because it can not advance the timing to get proper burn. So your cheaper gas is now providing you less mileage per gallon due to improper ignition timing, and your fuel mileage suffers. You now have to fill your tank more often, your valves, spark plugs, and (in the case of direct injection engines) your injectors start getting plugged up with soot from the inefficient burn of your low octane fuel, and the performance, and efficiency, of your engine further decreases. Now, your car, which may have made 250 horsepower and gotten 30 MPG when new, is making at best 200 horsepower (remember that ignition timing issue) and is probably experiencing somewhere in the low to mid 20's MPG. Not to mention, your spark plugs need to be replaced more often, you are probably in dyre need of injector cleaner to make them run better, etc...
Moral of the story is: you can run 87 octane if you want, but you are better off running the proper fuel for your engine.