It's only 1 exam and believe me the knowledge you gain in your BAK and PPL studies will help you with your CPL subjects.
As many have mentioned you will also not be able to take mates along for command building exercises.
Another downside is you WON'T be able to do NVFR rating (correct me if I'm worng) as you won't have a license to endorse, or at least exercise the right of a NVFR.
Also, if your class one medical lapses you won't be able to exercise the rights of private license as you do not have one (again, correct me if I'm wrong).
And lastly, you won't be able to do your IREX exam until you passed your CPL flight test.