The Security Council will be most unlikely to obstruct a recognised regional security organisation from doing its role. Any veto would be extremely costly for the power that did so as they would incur the ire of most of Africa.  Most of thesituations which the AU will want to intervene in are not of a high enough priority for the international community to devote significant resources to, but which are still big enough to seriously damage African countries. There will be no reason to veto. Moreover, there will be times when the world will be just too ignorant and too slow at some urgent matters which need immediate attention. A well-prepared standing army would have been able to stop the Rwandan genocide.  As a result, even though this policy is not perfect it would help the current situation in Africa improve.