Many people believe there are no risks if their cat is strictly indoors. However, occasionally bats will be found in a house and a house cat may encounter one or even try to play with it or kill it. Bats are very well known to carry rabies. Also, if your cat were to bite someone a quarantine would be imposed on the cat if it were not current on a rabies vaccine.
As for the other cat diseases commonly vaccinated for, the risk of infection is relatively low for a strictly indoor cat and no risk for human infection. There is still a possibility of infection if the cat were to accidentally escape outside or for the diseases to be tracked in on contaminated shoes. With all these factors in mind, our clinic highly recommends all cats to be vaccinated for rabies. All other vaccines are recommended based on likelihood of exposure.