I'm of a belief that it is better for a P plater to have a "nice" car (doesn't have to be powerful, fancy or sporty) - a car that is straight, clean and not a sh*tbox. Reason being it will force them into getting into the habit of taking care of their car, other people's cars and driving safely.
I've seen too many P platers driving around in sh*tboxes and not taking care eg. car parks and driving dangerously - I'm guessing because their mentality is that the "car is crap so why shoud I care".
Where as P platers with much nicer cars tend to drive more carefully (or at least make a genuine attempt).
Don't get me wrong - P platers will still get into accidents, but the ones with better cars do try their best to not get into trouble. Of course there will still be exceptions.