I had a collie who died of cancer of uterus from not being spayed. (Long story, totally the vets fault for thinking it was just a UTI)
And if I hadn't neutered Remi, his undescended testicles may have turned cancerous.
I heard a horror story once about a chick bringing her beautiful poodle into a dog park IN SEASON and before you know it, she was mounted by a pit bull (who wasn't neutuered). I wonder what the babies looked like. #stupidstupid
EDIT: But OMG I totally just read the bazillion paragraphs on other pages. Needless to say, my decision was 75% a behavioral one.