My opinion- Population control! There are so many animals who go hungry because no one wants to take care of them. I think that if can't/don't take care of the animals then you should make it where you're not put in a bad situation, and you're not putting helpless animals in bad living situations.
it's unfortunate we have to do it, but it's the only way to control hunger and overpopulation; i can't think of a better alternative.
If you can safely contain your critters and prevent accidental litters, there's some decent arguments for leaving animals intact. It DOES decrease the chances for certain types of cancers in large and giant breeds. Thing is, most people are NOT up for dealing with that level of vigilance. Spaying and neutering are just much easier and safer for everyone.