For me, leaving your pet un-spayed/un-neutured is the same thing as careless breeding. If you own an animal, you have no excuse not to get them fixed in the same way that you have no excuse not to give them proper vet care and nutrition. There are plenty of spay and neuter clinics around. If you can't afford it, you shouldn't own the animal and let it run around and probably get bred.
For instance, when I had guinea pigs I bought a new one from a pet store (never gonna do that again) and guess what? She was seriously emaciated AND preggo. I had asked the store owner if she could be pregnant when I got her and the lady said there was no way since she separated them (she had my girl and her 2 brothers) before they were old enough to breed and i didn't think to ask how old she thought they had to be to breed (turns out she thought it was 6 months when it's actually 1-2 months).
Well my girl popped out a baby boy and a baby girl. And guess what I did? Did I say "they're just guinea pigs, I'm not gonna spend the $60 to neuter the boy so he can live with the girls and have good quality of life"? I did not. I was only an 8th grader with no job but I did tons of chores and somehow made $60 so the boy could be neutered.
Along with making that piggies life easier, this last winter when I needed to rehome them all, it was way way easier to find him a great home (that I'm positive would never breed, even if they could) than it would have been if that boy was still intact. He was a "very rare" color and so he would have attracted a ton of wacko's wanting to breed for color even though he had no pedigree and even though he was inbred since his grandparents were siblings and his parents were siblings.
I know nothing about rats but I did have a petstore guinea pig (she was a rescue from a friend who was moving) live until she was 7 (freaking ancient for a piggie, their lifespan is 4-6 at the most usually.) She did have the absolute best of care from when she was around 2 (I joined a guinea pig forum and learned so much, lol) until her death. She would be still alive too except that her uterus decided to grow into one massive tumor and at her age surgery would have been very dangerous if not deadly.
At the same time, I had another girl piggy and her daughter (from my friends little sister letting the piggy "play" with a boy piggy,) mom from the petstore, baby was the product of 2 petstore pigs, that both died of neurological stuff around the age of 2 1/2. I'm pretty sure neurological stuff comes mostly from genetics so I'd bet that the mass breeding for petstores didn't help those two get a leg up on life. Haha
I considered spaying my girl piggies too but that is an extremely dangerous procedure for them and out of the 3 I had it would have been highly likely that atleast one would have died in the process. If spaying was less dangerous for them I definetly would have done it and had my little herd be comletely hormone free. Haha
Sorry that this is so long but it just makes me SO mad when people act like small animals aren't worth much. There are so many small animal rescues out there that are stuffed full with guinea pigs, rats, mice, ferrets, you name it and people keep breeding.