A private plate avoids "keeping up with the jones's" and prevents people from judging you on how new your car is.
Funny, in my eyes it marks you out as thinking you're ahead of the Jones's. As an example there was a family going through school about roughly the same time as I was and as each of them turn 17 they got a new car, with a private plate, with their initials on it.
I think what I found offensive in that situation was the psychology of not having to work for anything - stuff just being handed to you on a plate.
Oh and the fact that you need to stamp you initials on a car. Does your Mum still stitch in labels in your clothes with your name in, too? Or have you moved on to getting your initials monogrammed on to your shirt pocket. The Golf club is thatta way (and yes, I've probably ridden over the greens with my mountain bike).