Some rear spoilers on cars do help in terms of lowering the CD. Probably all of them help in terms of creating down force for handling. Whether they are worth the money they cost and the weight penalty (which certainly doesn't help mileage) would depend on the vehicle. Porche 911s are terrible without the spoilers. Their rear surfaces droop off too fast and the air separates way up without them. The Audi TT was dangerous on the Autobahn before they fixed it with a rear spoiler. The VW beetle could be greatly improved with a proper rear spoiler (not the silly little automatic lip at the top of the backwindow on their "sport" model).
When Ferrari first came up with the lip type spoiler on the back of their LeMans car years ago, everybody said it was just a "fix" for handling problems - until the windtunnel tests showed that it actually lowered drag too.
The belly flow on modern passenger cars is unimportant. A front airdam keeps the air from going under there in the first place and is easy & cheap. I know there is a Lexus ad where they turn the car over on a spit and babble about the dimpled undersurfaces but the improvement of that kind of thing is absolutely miniscule, if it is even measurable.
When I started in the industry, car aerodynamics were terrible so there was much to improve. Now the public is used to rounded surfaces, flush glass and high rear decks so much aero testing is aimed at handling issues and wind noise.
Allan