Spaying your female guinea pig is definitely recommended for more than just population control if she is housed with a male cavy. Females often develop uterine and ovarian issues including various cancers. Sometimes the uterus and ovaries can be removed even after the problem has been discovered but other times cancer has already spread to other parts of the body rendering it untreatable.
A complete hysterectomy SHOULD be performed by your exotics vet on your guinea pig at about six months of age to prevent uterine and ovarian diseases just as it would in a dog or cat. While some guinea pig owners are not able to justify the cost of the procedure because they don't recognize the importance yet. Others see the great benefit to having their guinea pigs live many years longer and not having to pay for emergency treatment when they notice their pig is sick, just like dogs.