Arbitrary, but better than other arbitrary measures (or, rather, the least 'bad' of the possibilities). Let's be honest, A-Levels are pretty indicative of your intelligence and/or conscientiousness, and for those who did badly at A-Level but got into a decent university and got a decent degree, the question remains: why did you not work hard throughout? For top law firms, ABB or whatever the cut-off is seems entirely reasonable: on AVERAGE, somebody who got less than that is probably not going to get a job at those kind of firms anyway. Yes, there will be some, but ultimately it's probably quite efficient.