On a street machine, it hurts the performance much more than it helps it
The main function of a spoiler is to create down force to improve traction. On a road race it helps in high speed cornering. On a drag racer it keeps the tires from breaking loose at high speed.
Neither of these is required on a street machine
Creating down force has a cost - Drag.
To overcome this drag requires horsepower. This is not a issue on a race car as there is usually lots of it. Street cars usually don't have more than a few hundred horsepower so there's not much to spare.
Most people today are putting huge spoilers on the rear of front wheel drive cars. This is the last place you need down force on this type vehicle. The rear tires don't do much for cornering. They won't break loose during hard acceleration. Placing down force on the rear of a front wheel drive car takes weight off of the front wheels where you really need it.
Anyone who has ever driven a front wheel drive car near its limits knows that they tend to push or slide the front tires during hard cornering.
Spoilers on a street machine are just eye candy.