A spoiler can either be designed to increase downforce, or it can be designed to reduce drag.
Downforce... this will increase drag, and add downforce at extremely high speeds if properly designed (which you would never encounter in normal legal driving).
Drag reduction... this will decrease drag, and hence improve fuel economy if properly designed. It usually works on Kamm-back style cars by causing the airflow to split cleanly from the car. Hence why you so often see spoilers on the eco variants of car models.
Aerodynamics is complex and easy to get wrong. Third party aero addons do not usually work properly unless specifically designed for a particular vehicle.
You will notice some of the "eco" models of some cars have subtle spoilers. A small thing can make a big difference, such as taping the shut gaps on your bonnet and bumpers too. Always makes me laugh, some of the huge square flat mirrors you see on American trucks, no doubt they use around 20% more fuel.
Well having your window down just an inch or two uses 5% fuel just by altering the aerodynamics. In the UK our driving instructors have large billboard type signs on the roof sometimes. These alone use 30% more fuel due to the extra drag they cause. If you look at American pick up trucks, one of the main reasons they get such poor fuel economy is the square shape and flat faces.
Worth noting that Spoilers on FWD's do help with stability at high speeds to keep the rear end of the car planted on the ground and stop it getting all skittish. Although you are correct in saying that on Front engine RWD's they make a huge difference to help aid traction that they're lacking because of not having the engine weight over the driven wheels.
Most spoilers you see are simply for looks. I would say a large majority of them are just for styling. If the car is advertised and known to be a performance sport vehicle then you're safe to assume the spoiler is functional. When racing air pushes on your front end which can lift your rear just a tiny bit to cause loss of traction in extreme racing condition. Spoilers are used to allow wind to force the rear tires down. In super cars the faster you go the more traction you have, kinda crazy huh? Haha My V8 mustang has a spoiler from factory. I doubt it serves any purpose other then styling. A Jet Cobra on the other hand is most likely a functional spoiler.
Easy question to answer. Spoilers are akin to aircraft wings attached to cars. As aerodynamic forces build, spoilers direct force down to oppose lift, which is what occurs when air rushes underneath the vehicle. Spoilers also delay air separation from the tail of the car which reduces drag by moving the low pressure zone behind a moving-car farther back away from the tail of the car. However, all spoilers introduce their own drag and are angled to achieve the right balance between drag and downforce.
You can simulate a spoiler's function by holding your hand out of the window fully vertical (maximum drag) and then angling your hand to approximate the spoiler's angle. You'll feel your hand being forced down and you'll feel drag diminish (when compare to holding your hand fully vertical). Unfortunately, aerodynamic forces don't really come into play until you are traveling at high speeds...lets say, in excess of 70mph (aero forces are always there, but the magnitude is small, even at 70 mph). So on most passenger cars, the spoiler looks cool, and provides a bit of high-speed stability and diminished overall drag.
Just for information, Porsches often have spoilers that deploy at 75 mph for the reasons described above. In addition to providing stability while in motion, the additional downforce aids high speed braking as well.
wether designed properly or not, unfortunately, any spoiler will add either downforce or drag to any car, wether it adds good or bad is a matter of design. the 500$ honda running around with the enourmous 3 deck rear wing, was not designed for anything but looks, but i garuntee it is "by accident" adding significant drag to his car... just thought id point that out to anyone who wants one just for looks.. chances are high, u will get more than looks