When I was learning about Niddah, I was particularly reminded of two different concepts. One was the myth of Achilles. Basically, Achilles needed to be bathed in a special spring to provide him immunity from all weapons so that he would be invincible and could not be harmed or killed. However, his mother held him by the heel, so that when he was immersed, that one part of him was not covered by the special healing water. Therefore, his weakness was his Achilles heel and that is how he eventually was murdered. This story also appears in Norse myth by Sigurd, who was bathing in dragon's blood that would make him invulnerable to all weapons and indeed make him immortal; however, a leaf fell on his back so that one spot became vulnerable to the touch of steel. The same concept, I believe, applies to our concept of chatzizah and how there cannot be one.
Secondly, I think that the two weeks of separation from one's husband leads to a very important lesson in understanding and compassion for our children, especially our toddlers. A child who has had his toy taken away from him, either unfairly by a sibling or friend, or deliberately by a parent, will get red in the face and scream loudly and unceasingly, "But I want it! I want my toy; I want my dolly!" This could be exasperating to a parent, especially if one has to sit through five hours of this. But when someone has to physically separate from one's spouse for a period of two weeks every month, they understand what their child feels like. They too have had their spouse unwillingly taken away from them by a higher power (God). They too feel like throwing temper tantrums and screaming, "But I want my husband. I want it." And thus this experience affords them compassion for their child and thus the needed understanding and patience to comprehend what the child is going through even as you realize the necessity of depriving him of that toy at that particular point in time.