Apostate prophet finds social merits; slavery is a chance to reform?

In answer to the video "Walking Away From Islam"

Another repercussion of the Islamic system of "slavery", is that when captured during a battle against Muslims, those prejudiced individuals whose aim really was to annihilate Islam by all means, suddenly find themselves under the guardianship of those they believe represent evil.

Instead of being mistreated or even killed as they would have done had they captured Muslims instead, they benefit from strict regulations that guarantee their well-being. These people who, like in our days, were brainswashed with a distorted image of Islam, were shocked at seeing and experiencing the truth behind the scenes, benefiting from it, and how just and fair a social system it was as compared to their own society and how they themselves treated their slaves and war prisonners.

They experienced the reality of divine justice and many converted. So by restricting the acquisition of slaves to war prisonners, Islam was (and is still in case war is waged on Muslims because of their religion) actually giving them a chance of reform by introducing them to a better system on all levels

8:70"say to those of the captives who are in your hands: If Allah knows anything good in your hearts, He will give to you better than that which has been taken away from you and will forgive you, and Allah is Forgiving, Merciful". 

The voluntary assimilation and conversion of war prisonners because of having witnessed Islam from within, parallels with what happened during the treaty of Hudaybiyyah. It provided a relative peace era on the Meccan front which allowed Islam to spread faster than it ever did, because it allowed Muslims and non-Muslims to visit eachother and interract on account of their family relationships and trade connections. Many Meccans started visiting Medina, and stayed there for months. They got acquainted with the teachings of Islam and were deeply impressed by the righteous conduct and moral integrity of the Muslims, and how they treated the prisonners of war, integrating them into the fabric of society. Islam gained many converts in its ranks during that period.

This sense of brotherhood which Islam gave the oppressed, helped gradually bring down social barriers.

As a result, in the early periods of ISlam after the prophet, we see countless slaves with high political responsibilities, including the command of armies, governorship and judgeship. Not only in administration, we find theologians, commentators of the Quran, traditionists, jurists and authors who either were slaves or the children of the slaves or ex-slaves.