There are many reasons to take steps to keep your anonymity, including hiding your car number plate.
Blackmail, all on its own, is rare, but does happen. Why take the chance?
If you're going to review prossies you are going to piss some off. Even with positive reviews.
What happens if a prossie gets mugged and accidentally gives the police your plate? Or gives it because she is trying to track down potential witnesses? Do you want that knock at your door?
The problem is that the consequences of being outed, for many, are huge. Taking basic precautions, which is all the vast majority of us are talking about, is absolutely paramount.
If you have anything to lose, and you're not at least a little worried about this stuff, then you're a fool