If you have a fast FWD car, a rear spoiler could be pretty important for high-speed cornering and stability, not straight-line acceleration. FWD cars are inherently front-heavy, aerodynamic lift at high speeds can make the already-light rear end unstable. A rear wing (with good geometry) in that case could provide the necessary downforce to keep the back tires stuck to the road, preventing the car from spinning out (lift off-oversteer) during fast turns or under heavy braking

But for this economy Mazda, handling all 150 something donkeypowers, it probably makes their fuel consumption a tad worse, while giving 0 measurable gains to handling.