Aside from them reducing the population of the Christians, it probably makes them less likely to rebel since they could be fighting their own offspring (not that their children would probably remember them). Not to mention this also reduced the number of males Christians can field.
If it was not disgusting, I would say it was a brilliant move. I wonder if anyone has used this AGAINST Muslims.
Islam is a religion which gives a lot of emphasis to the demography. They take every step to increase their population. The males can have up to 4 wives (including non-Muslim women), and the widows are immediately remarried. Also, they are encouraged to have as many children as possible. Family planning is strictly prohibited. In war, the non-Muslim prisoners of war are spared from execution of they convert to Islam.