Well, spoilers are needed in sports cars which basically focus on performance rather than FE. Spoilers (if properly installed) aid in the car's handling by generating enough down-lift to press the car down when moving at high speed thereby improving the tyres' grip. Spoilers are useful only at high speeds (the kind encountered during racing). They add to the vehicles' weight (small, but its extra weight nevertheless) and they introduce extra drag on the vehicle (again small in magnitude, but its a negative effect nevertheless). For safety at high speeds, both these negative effects are tolerable - and where speed/performance is paramount, FE is not that important, so spoilers are desirable.
However, for ordinary passenger cars they are nothing more than an ornament - though they will still provide some downforce (if properly installed) at high speeds, they will actually end up degrading the FE, albeit by a small percentage (due to reasons stated in the last part of the preceding para). Note that passenger cars usually don't move that fast (and are aimed for economy rather than performance) and hence don't come pre-fitted with spoilers - the manufacturer does not consider them important.
 S = k.I^2, where S is the amount of stupidity a species possesses, I is the intelligence the species has and k is the universal constant of stupidity.
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