You guys are great.
As it has been alluded to above, it's a trade off; to generate lift or downforce your engineering compromise is C_d (or even your frontal area, which also increases drag).
In most driving conditions you're not limited by traction (where downforce is necessary), hence, you're just stuck with the drag tradeoff and none of the benefits. Every aerodynamic device's performance is a function of Reynolds number...translating into performance being "optimum" at one relative velocity and not at another. Proper use is heavily dependent on application...nothing general purpose here.
But hey, glue all the wings you want onto the back of your pickup trucks. It's the visual statement that matters
