We know from Judges 21: 14 that occasionally there were shortages of women. In that situation, to allow captive women into Israelite families in order to procreate would be an expedient measure. Josephus in his discussion of the captive woman (Antiquities IV: 258 ) mentions that when a man takes a woman in order to have children by her, he should be considerate of her wishes. The fact that he mentions this directly in relation to the captive woman is significant.
It can be construed that we see a tension biblically between the fear of theological pollution and the occasional need to replenish the breeding stock, whatever the reason. This might explain the inclusion of the law allowing captive wives. It would seem that the effect of Deuteronomy 20: 16,17 was to ensure political and theological continuation, while the effect of Deuteronomy 21: 10ff was to help biological continuation by enlarging and diversifying the genetic pool from a non-threatening source.