"Form follows function" cars tend to not sell good. The previously mentioned Audi A2 is a good example of this, it had aluminium chassis which reduced weight + quite good aerodynamics if i remember correctly. It is quite efficient for a car in its class but it't didn't sell because of it's looks.
For ordinary people the looks of the product they're buying is more important than the benefits it offers them, as long as it does what it's meant to do. This is one of the reasons the ordinary cars have terrible aerodynamics.
I had a tour in the Volvo factory a few months ago and the guide explained the various parts that make their C30 more efficient, splitters, spoilers, strange wheels etc. But she also mentioned that their design team had the last word not the engineers. For example the slope of the rear window is 30 degrees, if I remember correctly, something the engineers did't like very much but the designers had the last word and went with the 30 degree slope which has terrible aerodynamics. If Volvo would've followed their engineers they would've ended with a crappy selling A2 clone.
For supercars it's not the looks that come first but the brand. Most rich people are uneducated when it comes to cars so they buy the car because of what they THINK it can do and the status it gives them, not what it actually DOES.
To illustrate this i'll take two brands from the electronics industry but i won't mention names. Brand A puts more effort in the design and feel of their products, which it then markets as something out of this planet and prices accordingly. Brand B on the other hand produces products that are more capable but lack in design and UI. The price of brand B is at least 25% lower. Which one sells better? Brand A off course! Not because of what it can do, but because people think it's the best!
I would buy the McLaren without hesitation but I think that the SLS will sell better because of the above mentioned reasons.
ps. sorry for the long post