It doesn't necessarily get you an unroadworthy nail. Spent correct it'll get you a car that will get you from A to B, fundamentally the same function that this £10,000 Seat will perform, just without the crippling debt that the young lad WILL come to regret (probably as soon as the '11' plates hit the street and his car is no longer new).
I'm not implying in any way that a £100 car is a viable alternative to a Brand new Ibiza, but I don't see how pottering about in a hundred quid car is anywhere near the 'fail' as lumbering yourself with a considerable debt at 17 years old in order to drive a rather rubbish car.