My bold - how's that for an ad populum argument?
In the case of your thick friend, this rule would not help him (unless everyone else who applies is even more thick, but fortunately the job is such that a vegetable could perform it after the available training).
Comments about middle class are interesting though. I sat 9 GCSE's and got all A - C, from a 'failing' comprehensive which is defined as one where fewer than 20% of pupils get five A - Cs (or fewer than 20% of individual results are A - C, I forget which).
IIRC you have a similar background.
Now let's imagine Tarquin, who had every advantage money can buy all his school life, went to a fee-paying school and had additional private tuition in the holidays. Tarquin was pretty thick though and only got exactly the same GCSEs as me (or you). Tarquin applies for the same job as me (or you). Both applicants have the same qualifications, but who is likely to be the better candidate? The one who was tutored up to the eyeballs and stretched to fulfill his potential completely, or the one who had crappy resources and other worries to contend with?
Considering that specific minorities are overwhelmingly likely to have had a paucity of education (series 4 of The Wire! Easier to apply this in the US) compared to their leafy suburban counterparts there would seem to be an argument in favour of actual, explicit positive discrimination* in some circumstances, isn't there? Given that it's facile to chirp on about how everyone should have the same opportunities regardless of colour and blah blah blah, should we not recognise that inequality is a very real and current problem, and it will remain entrenched until someone actually does something about it?
*Of the sort that absolutely was not proposed in the UK....just to avoid any further confusion....