D&C 132 says sometimes concubines are ok. How can this be just?

We have to understand the context of concubines/servants outside of our culture’s paradigm.

Most people think it’s bad to have a servant (some call them slaves), yet I choose to be a servant of The Lord, and follow his every command.
The most effective, armies are those who follow orders even when difficult or dangerous.
Many people in our society choose to be employed by someone, doing everything they say, because the employer takes good care of them, and it’s a voluntary system.

It seems there is a right and a wrong way to do this, like anything else.
WRONG: A harem, only used for sex, doesn’t contribute to family life, no offspring, at taxpayer expense, beatings and other poor treatment.
RIGHT: Authorized by priesthood authority, voluntary, treated as a family member, loved, contributing to family well-being, offspring from concubines learn to love the family organization, feel blessed in it, and chose to remain with it even as adults.

Perhaps these concubines could eventually be elevated to wife status, unless they liked where they were, the jobs they were doing. After all, not everyone wants to bare the full “weight” of glory, it’s accompanying duties, etc. Some enjoy simple work more than complex work, etc. We shouldn’t think everyone wants the same job.
Some get close enough to a leadership position that they realize they are thankful someone else carries the weight that comes with the job.
Remember, wives work too, they’re just more in the managing teaching and directing than the concubines.

The place for a concubine is outside the inheritance line. Other then that, they are as wives. And ideally they could advance if they wish at some point.

Marriages would unit nations/families. Where both have inheritance.
Children of concubines would be servants to the joint inheritance.

So servant status rather than full family status.
One of their purposes being to raise seed to the family kingdom to run things.
But they were treated with love and respect.

Sons of Zilhah & Bilpah still fathered their own tribes, but they did not receive the greater blessing. They still have an awesome posterity, even comparable to Benjamin’s.

Let’s say you and I each had a large family farm. And we wanted to unite our farms, our oldest children would marry and become joint heirs to the combined property.
If there were other children, that divided the inheritance then with would weaken instead of Strengthen.
So, we say our two heir apparent has the inheritance and everyone else serves them.

Many of God’s children haven chosen be servants to Great and noble men in women in God’s kingdom because they are just thankful to have a seat at the same table as them.

Imagine being a servant to Abraham as a child of a concubine. You get to be personally mentored by him and then learn to be like him.

Naturally this is not fully understood at this time (by me leastways). Perhaps this won’t even be necessary eventually and was useful more in past than future.

But the fact remains that it has been commanded of God historically, and can be a compassionate and positive system for all involved.