In my opinion it's worse to have the rear spoiler and no front spoiler than the opposite. Still having a front spoiler only isn't ideal either. The subject is thoroughly discussed at
www.netmotor.de although in German rather than english.
I have the front spoiler only (for cosmetic reasons) and have been up to about 125 mph since I got it. Couldn't feel any difference compared to earlier when I had no spoiler.