Properly designed spoilers can reduce both lift at the tail and drag. They do this by "spoiling" the airflow as it goes down the rear of the car. This helps the air that goes over the top of the car and down the rear to cleanly separate from the vehicle. Otherwise it creates drag and lift inducing vortexes and eddies at the rear of the car. This is why gunk and grime get deposited on the rear of a car driven in rain - the air is rotating behind the car and creating a negative pressure area.
The Beetle (both old and new) has a poor shape at the rear, in that there is nothing to encourage this airflow separation, so a spoiler is especially helpful on it. Notice also that Porsche has through the years added many different kinds of spoilers to the 911 and made the tail progressively taller to remedy this problem. A long, long tapering fastback is in many ways the perfect shape for a car's rear, but in the real world it would be impractical to have such a long tail and in any event skin friction between the air and long tail would begin to outweight the advantages of it. That was the genesis of the Kamm tail, and the principles still hold true today.
I would wonder if the revised for '04 trunklid on the Jetta sedan helped the drag a bit, in that it has a small lip at the trailing edge which should promote airflow separation. It often doesn't take much to do the job at normal highway speeds.
