The risk of reproductive cancers for an unspayed female rabbit stands at 85%, and is virtually eliminated by spaying your rabbit. Your neutered male rabbit will live longer as well, given that he won't be tempted to fight with other animals (rabbits, cats, etc.) due to his sexual aggression.
They are calmer, more loving, and dependable once the undeniable urge to mate has been removed. In addition, rabbits are less prone to destructive (chewing, digging) and aggressive (biting, lunging, circling, growling) behavior after surgery.
Un-neutered male rabbits spray, and both males and females are much easier to litter train, and much more reliably trained, after they have been altered.
Over 15 million adorable dogs, cats, and rabbits are killed in animal shelters in this country every year. In addition, unwanted rabbits are often abandoned in fields, parks, or on city streets to fend for themselves, where they suffer from starvation, sickness, and are easy prey to other animals or traffic accidents. Those rabbits who are sold to pet stores don't necessarily fare any better, as pet stores sell pets to anyone with the money to buy, and don't check on what kind of home they will go to. Many of these rabbits will be sold as snake food, or as a pet for a small child who will soon "outgrow" the rabbit.
Rabbits are social animals and enjoy the company of other rabbits. But unless your rabbit is altered, he or she cannot have a friend, either of the opposite sex, or the same sex, due to sexual and aggressive behaviors triggered by hormones.
A knowledgeable rabbit veterinarian can spay or neuter your rabbit with very little risk to a healthy rabbit. Don't allow a veterinarian with little or no experience with rabbits to spay or neuter your rabbit.