ICould I agree with everything you've said about being a brood mate, I'm sure their children are very much wanted and loved. I don't think William married her as a brood mare either.
But come on, they've never been a great romance have they? William was one of the most reluctant grooms in the world, he practically had to be dragged down the aisle, they haven't had a plain sailing relationship and they've never looked particularly in love, happy or at ease together.
I don't think for a moment he married her as he had to marry a suitable brood mare as Charles did Diana (and I don't think anybody can deny she was a brood mare). But I do think he felt he had to marry Kate because he was expected to marry a certain type of woman as a direct result of his mother.
Kate is cooperative, pliant, does as she's told, doesn't rock the boat or hog the limelight. She's not independent and she doesn't have many strong opinions or big ideas. I suspect William felt under pressure to marry her because she fitted a suitable template a la Queen Alexandra which was very appealing to the royals after Diana.