If other species of animals be properly trained and are able to fulfill the functions of a service animal, they should absolutely be able to become service animals. Increasing the number of species available could open up opportunities for animals with different skill sets and strengths to perform a variety of tasks and duties.
More species of animals should be allowed to become service animals if, and only if, those animals can be properly trained and declared safe for use. Who cares if the animal is a dog, cat, monkey, or kangaroo? As long as it serves a purpose of helping the person in need, why not?
More and more we see in the news about how a vast variety of animals are being used as service animals. From a cat in England to organizations that train Capuchin monkeys, the need for animals to aid humans is expanding beyond the limitations on what just a dog or a cat can provide. People with disabilities need these animals in order to live their daily lives and an animal is treated in a kind, humane way and is given the chance to have a supportive, loving home.
Service animals serve a critical function: they help the blind and disabled navigate a society that is, unfortunately, not built to support them. However, as appealing as the idea of other species stepping in to fulfill this task may be, let's only let dogs (and occasionally cats, who can be helpful therapy pets) be service animals. Dogs are uniquely equipped to intuit human emotion. It's for this reason that they're called "man's best friend" - and why they are so good at being service animals.