Thing is that with lower octane fuel it will not burn as efficiently. So while you maybe saving $2-5/fill up, you have to fill up more often costing you more in the long run. It is simple, period. Plus, you can so tell the difference when going from 87 to 91+. My first car during high school (1990) would get the 91 octane ($1.20/gal) treatment on payday and she was happy for a full tank. Then it was back to 87 ($.95/gal)...
"With great power comes great responsibility." -El-Superbeasto