The rear spoiler does change the aerodynamics of the vehicle a little, mostly by blocking/reducing the amount of turbulent wake which tends to climb up from behind the car onto the trunk lid behind the rear window. By doing this, it reduces drag a little. Also because of that it does reduce the aero lift at the rear of the car a little, but not because of it being a wing in that respect. Redirecting and smoothing the airflow over the top of the car and back, ie, reducing it's turbulence on the trunk lid which does the reduction.
The front lower spoiler also tends to reduce the drag a little by reducing the amount of air being rammed under the car, which if not blocked tends to cause excessive drag. This also tends to reduce the amount of front end lift. Basically, cars as generally designed tend to create lift which in turn creates an induced vehicle drag. Reduce the lift, and you reduce the drag.
Whether or not the car is front or rear wheel drive doesn't have anything to do with that phenomenon.
To quote the S Owners Manual, Alfa does say that the S aero fairing package does reduce drag and lift of the overall vehicle (I suspect slightly). They do show a pressure distribution side profile of the S, but not of the standard 164, alas. So, can't really compare, but I've seen many vehicle lift and drag test results to confirm it in my mind. Used to work compiling wind tunnel test results of aircraft and vehicle models, and once in a while, buildings models, even the Space Needle, and years before, Galloping Gertie, the infamous Tacoma Narrows bridge.
Vehicle aerodynamics is really fairly tricky, and manufacturers spend a ton of money to improve their products, often in very subtle ways. I do remember something from Alfa years ago, where they took a Sprint GT/GTV and tested it on the road while instrumented. First they tested the original as produced design to get baseline results, and then the added front and rear spoilers to various extents. They reported that while drag was reduced a little, the vehicle lift was significantly changed, 300 lbs less lift measured in the front suspension at 100 mph, and 150 lbs less in the rear. The original aftermarket Bobcor front spoiler for Sprint GT/GTV's was probably a copy of those Alfa spoilers, looking similar as I remember. I have this testing report somewhere, I'll have to search for it.
And yes, it is also for looks, lol. After all...
In that respect, these are not full race cars, where aero is all important and negative lift is achieved by much more involved appendages. Witness the ultimate, F1 cars, where different teams may have the same engine, but the aero designs can make much of the the difference between being a consistent winner and a consistent middle runner. Some teams figure out the aero characteristics of their chassis, some do not.
Finally, I put an S spoiler on the trunk lid of my LS in order to break up the look of that big bluff trunk lid. I just happen to think that unadorned lid looks a little big and bluff, esp in the light champagne color. Works for me, anyway.
And really finally, I like both cars. The S is a good around town and winding road car, and the LS is a great highway faster swervy roads car.