Your manual should be specific about whether 93 is "recommended" or "required" - there's a crucial difference. For the vast majority of cars these days, onboard electronics are programmed to compensate for lower octane fuel, technically making 93 octane obsolete for all but the highest end of sports cars. Various tests by car mags and consumer organizations have confirmed that lower octane fuel has no/neglible adverse effects (performance, wear, emissions or mpg) on the vast majority of new cars.
That said, I could've sworn I felt a slight performance difference when using 89 and 91 fuel in my wife's Audi (a 3L supercharged engine with 333hp). It was ever so slight, but I could've sworn I felt a tiny difference. That combined with unfounded doubt in my mind, made me switch back to 93. I figured the marginal incremental cost annually of 89 vs 91 vs 93 didn't warrant the lower grade fuel. I only use 93 in the M5, but would be comfortable with lower octane fuel in a non-M BMW if BMW only "recommends" 93.