Most likely it is reducing drag. Small lip spoilers like that are generally put there to help clean up the air flowing off the trunk. The only way to really know for sure is to
1. Talk to a honda aero engineer that worked on the hch
2. put the car in a wind tunnel
3. get a 3d model of the car and use CFD (computational fluid dynamics, program like Fluent) to analyze the effects of the spoiler on the boundry layer over the rear of the car. These programs can do wonders for aero engineers, once you have a model you can see exactly where your turbulent areas are.

You'd be amazed at the amount of aero work that is put into cars these days. The under tray helps maintain a cleaner flow of air under the car. This is why most cars these days are starting to use those under trays. You will also see a lot more rear diffusers on sportier cars, they not only clean up the air flow but help reduce the low pressure area behind the car. They are generally used to help create negative lift (aka downforce) on performance cars. The dirty trunk is most likely a result of the "dirty" air behind the car.

06 HCH Mag. Pearl w/o Nav
05 wrx STi