God likes his vessels young and tender. Joseph Smith, for example, was (according to LDS Church speak) an innocent, uneducated young lad of only 14 who couldn't possibly have known the ways of the world. That's important for us Mormons to believe because it's an example of the way God works. He took something weak but pure and made it strong! That's the kind of generous god we worship.
So is it any wonder that God commanded Joseph to marry a girl "several months before her 15th birthday" (OK, more than one actually)? We all know how strong these chosen teen brides (OK, there were a lot more than just two) became, right? I mean, they were pivotal in the highest ordinances revealed to Joseph! No?
I guess being shuffled about like chattel in life only to be completely swept under the rug after death and then dredged up only reluctantly because you're a major liability might not feel especially great.