In my view discrimination is wrong, so therefore surely positive discrimination is also wrong? For every person you give a 'favour' to your shafting another poor sod who might have worked three times as hard.
I think it's a ridiculous system personally. It might also remove any sense of, 'Well, I really worked for and earnt this.' in those that were rewarded. As of course any black person who does get promoted might think at the back of their mind that really it's worthless, as there's a very strong chance that he was just given it because of the colour of his skin, and that can quite easily be seen as an insult. With that in mind he or she might also then feel that the military does not value his capability at his [or her*] job, or the work that s/he has put in and merely used his promotion as a PR campaign or part of a political stunt.
If you saw your promotion in the newspaper under the headline, 'equality in the Armed Forces: Navy promotes ethnic minorities.' How would you feel?
The best idea would be to have no discrimination at all; after all, it'll make someone feel uncomfortable one way or another.
*For example I feel things like His/Her are ridiculous, let's just have the male form as neuter and be over and done with it. It can barely be seen as sexism if you don't write the female form, surely? Man-Hole Covers are now Maintenance-Hole Covers on the BBC, or something ridiculous like that. Police-Person.