The reasons for spaying/neutering rabbits are pretty much the same reasons we spay/neuter cats--for health and behavioral reasons, besides birth control. Females have something like an 85% chance of getting reproductive cancer if not spayed, and both females and males will spray urine like a tomcat to attract the opposite sex. Unneutered males will fight with each other, frequently causing serious injury. If you want to keep them indoors, altering will make them much more agreeable housepets.
Plus, I procrastinated too long in having one of my rabbits spayed, and she seemed to get "broody", I guess you'd call it. She pulled all the fur off her tummy, wouldn't eat, and kept digging nests in my carpet. She just wasn't happy at all. She's much more content now that she's spayed. Additionally, rabbits are social creatures, and aren't happy alone. Obviously, spaying/neutering will make group dynamics much easier, and make male/female pairings possible (without ending up with tons of bunnies!), which is good because male/females pairs tend to get along better (like cats).
The exotics vet who spayed my bunny actually charges $2 less for a rabbit spay than a cat spay (prices around $100, more than my dog/cat vet charges). He does a really good job, though, on cat and rabbits.
For ferrets, altering is absolutely necessary. Intact males stink so bad it's hard to be in the same room with them, and have a lot of disagreeable habits. Females stay in heat until bred, and if they are never bred will eventually die from aplastic anemia. But most pet ferrets are sold already spayed/neutered, so it's not something most ferret owners need to think about.
For the smaller pets, the main reason to spay/neuter is for birth control, or to make it possible to keep multiple males in the same enclosure without fighting. Some species are social and can become very attached to each other (not hamsters!), so breaking up a bonded pair to prevent babies can be very stressful. In a case like that, I'd neuter the males, and not put the females through such a traumatic surgery.
ETA: since female rats also have a very high rate of reproductive cancers, for a while routine spaying of pet rats was widely recommended. Unfortunately, it didn't seem to make any difference in their cancer rates.