The manuel does recommends using 91 octane in your tank. Like others have said regular gas wont kill the car but it will run better on 91 octane since that is what is recommended by the owners manuel.
While I understand what you are saying and what you are saying it true, smart themselves recommends using at least 91 octane. I cant find an owners manuel on hand but here is a link from smarts usa detailing to use 91 octane for best fuel economy.
They say the car can run on regular fuel but you will get better gas milage on the higher octane.