As a comment that is neither pro nor con spaying, I live in an area where ranching is the predominant industry and horses are still very much used. In our corner of the universe, mares are preferred over geldings for a go-all-day, I'll-give-it-my-all sort of horse. One of my good friends, whose family has an unusually large cattle operation, in terribly rough, broken country, uses mares almost exclusively.
They spay nearly all their mares. In my friend's opinion, you get all the benefits of a gelding - steady mood, predictability - with the additional benefits of a mare - which is seen as loyalty, grit and heart that surpasses a gelding.
Talking to him, he'll pick a mare over a gelding any day as a solid using horse. Not something I initially expected from a big, gruff rancher-type.
I'm not saying his opinions are correct, but it's representative of the common thought here. I've never heard of any of his mares having difficulty or dying from the surgery.