I think that a women quota is useful in cases where the proportion of female (one could also insert specific cultural backgrounds here) possible applicants for a role (be it junior or leader) is much higher than the proportion in the actual role. For instance, in theatres or hospitals in Germany, a large part of the employees is female whilst the bosses tend to be male. You cannot argue with "but better candidates will be turned down just because they are male" because obviously, the criteria for choosing are heavily unbalanced already. It's harder to determine whether a quota is useful in jobs the reason for not choosing women might be women not applying for the job. But we won't solve the matter by hating each other over the cause. Let's try to be critical, yet empathic.