As you have probably already read, this allowance created a way for female slaves to obtain their freedom. As far as I have read, I have come out with the impression that Islamic law mostly, if not always, follows a favourable risk-benefit ratio, meaning allowing things which have the potential of creating more good than harm in the society and prohibiting vice versa. Such, I believe, is the case for this allowance too.
A female slave is already guaranteed food, shelter etc., but there still remains the issue of her possibly remaining a slave for her entire life (in case other Islamic methods of emancipation aren't used), and her children also being born as slaves. If she has children of her master, she is guaranteed freedom on her master's death and her children also, if I remember correctly, have inheritance rights ensuring she and her children are looked after.
On the matter of "... husbands they loved...", if they love them then they could remain abstinent, there is no evidence that Islam allowed them to be forced into this. One of the things that has to be remembered is the context of those times. If a woman gets captured in a foreign land, there are many problems with her being able to again meet her husband. Distances are months long, rumours of death can spread, news is difficult to reach, the husband could have died or just have chosen to forget about his wife. Therefore, such women would have had a lesser probability of obtaining freedom as compared to others.
In such scenarios, there is some benefit for this allowance and, if the master is God fearing and follower of God's restrictions, there is no risk of this allowance being misused.
