One very important thing to consider is that: like their human counterparts, the hormones created within the reproductive system that spaying/neutering remove, are needed for more than just reproduction. Some are used to help balance other essential hormones, some help with muscle regeneration and repair, some regulate temperature, metabolism, bone growth and repair, organ function, etc... Think back to your high school biology class when you had to learn about the immune system and all of the important pieces and parts that help maintain optimal health, the hormones produced by the reproductive organs are included. They are also an important component in the "emotional" part of the brain for mental stability.
Also consider what happens when women under go menopause, either due to hysterectomies or naturally, because when you remove the reproductive organs in your female dog, you are doing the same thing. Osteoporosis, other hormone fluctuations that cause mood swings (bitches that start acting more "male"), decreased immunity, slower metabolism, fatty deposits in different places (more around the middle vs distributed evenly), etc..
Spaying does not remove the entire risk of developing mammary cancer, which is also a low risk even among unspayed females and if caught early, easily removed. Yes, dogs do get cancer and not all survive even with the best medical care, but you really have to weigh the odds and risks involved before deciding to remove necessary organs that play such an important role in your dogs' lives.
Spaying DOES help with preventing pyometries spelling? , but they can still occur if the "stump" of the cervix left behind becomes infected.
If I had a female that, for what ever reason, I decided to spay due to not wanting pups or the "hassle" of heat cycles; I would wait until she was at least 2 yrs old or had at least 2 cycles for her to fully mature (mentally and physically) and then I would consider leaving the ovaries and just have the uterus removed. But that is what I would do, you have to make your own decision based on your wants/needs/dog.