A+ for effort, at least
Nothing changes that the combat is one of the most gamiest I have ever encountered.
It simply does not feel right to - against all common sense - leave an enemy that has not lost any of his threat alive.
You do it, because you know it is effective. But you know you would never do this in any other game or a comparable "real" situation - "real" as in "if you were one of the guys fighting there".
Now, I'm not saying a fresh breathe of air in combats like that isn't welcome.
But there would have been ways to implement this far better.
If units would in any sense become weaker when low on HP, this mechanic would suddenly make far more sense.
No need to deal with weakened enemies when there are bigger threats to tackle.
And the argument that turn based combat is not realistic to begin with is just a straw man. Nobody in their right minds would ever claim turn based combat is realistic.
However, it serves as an abstraction of a realistic situation (again, realistic as in realistic within its setting). So, the closer to that realistic situation it gets, while still being playable, of course, the better.
And in such a situation nobody would go "Okay, now we need to make sure this minion doesn't get killed so his brethren don't get stronger". Wtf?
In the end I think this comes down to what kind of player one is more.
IMO, there are two extremes:
There are people who play a game, who dive into the lore, the world, who (to whatever extent) roleplay and identify with their characters, etc.
For those people, immersion is most important and everything - including the mechanics! - has to make sense within its context.
And then there are people who game a system, who take the rules apart, analyze and find optimal solutions, who want to "beat" the system as good as they can, not caring much about the worldbuilding. What is most important for those people, I have no clue, but it sure as hell isn't that something makes sense within its context.
For Banner Saga, the combat sure is for the second category of people. Though I am somewhat convinced the designers were not even aware of that.