DCX knows what they're doing. That's why they chose an octane that will work for all. With all the variables involved with vehicle octane requirements, 89 is the safe, and conservative compromise for all. This is not to say that lower octanes can't, or should not be used. That is why they use the terminology, "recommended". Those that live in cooler climates may be able to use a lower octane fuel in the winter. (See last reference). Owners that live at altitude do not need, nor require the higher octane (a waste of money), as stated in the following references:
Gasoline Digest, "In high-altitude areas, you’ll find lower octane gas. “Pressure and temperature in the combustion chamber are less at altitude.” Wusz stated.
Gasoline FAQ, "On modern engines with sophisticated engine management systems, the engine can operate efficiently on fuels of a wider range of octane rating, but there remains an optimum octane for the engine under specific driving conditions."
"Once you have identified the fuel that keeps the engine at optimum settings,
there is no advantage in moving to an even higher octane fuel. The manufacturer's recommendation is conservative, so you may be able to carefully reduce the fuel octane."
"If you are already using the proper octane fuel, you will not obtain more
power from higher octane fuels. The engine will be already operating at
optimum settings, and a higher octane should have no effect on the management
system. Your driveability and fuel economy will remain the same. The higher
octane fuel costs more, so you are just throwing money away."
"You may be able to change octanes between seasons ( reduce
octane in winter ) to obtain the most cost-effective fuel without loss of
So, not everyone needs, nor should necessarily use 89 octane fuel. As stated in the references, find the one that works best for you, in your environment and climate, and go with it. Some may be able to change octanes from summer to winter. Personally, I've used 87 almost since I got my Magnum RT, and although the per tank savings is not that much, figure the savings over 15000-30000 miles at today's prices, and the savings are no longer pennys. Using 91 or 93 anywhere would definitely be a waste of money. Also, those who must use reformulated or oxygenated fuels will probably see lower performance and lower mileage.