Seems to me like it would increase wind resistance with no benefit. They do have a nice "jet intake" look to them but not nice enough to put on my DD just for the eye appeal. Would look great on a show car.
Edit: Plus, it's hard enough to see out that side as it is. Insanity to block even more of the view. Great for a show car but not the road.
Edit 2: Egads! You have to roll the windows down to open/close the doors! Forget to roll the window down (I can see that being very easy to do, especially for a passenger) and you rip the cowls off or break the window.
Edit 3: Not true. The cowl does not extend forward enough to interfere with opening/closing the door. Thanks to
KaienZ34 for pointing this out.