Here is a question I often get: "Dr. Jon, I know why you must give an outdoor cat the rabies vaccine, but why does an indoor cat need it?"
The main reason we vaccinate ALL cats is simple. It's the law.
Both indoor and outdoor cats are required by law to get the rabies vaccines. Every state and county requires every pet to be vaccinated for rabies.
At first, this may seem illogical, but it really is a good policy. Even indoor cats could have a risk of getting out and getting bitten by a rabid animal (although I admit the risk is slim). An additional benefit is that IF your cat ever does bite someone, you and your cat will be in compliance with the law since your cat will have had all of the required rabies vaccines.
Here is a good article on this subject. Go to: What vaccines do cats really need
Here is another very good article by The Irreverent Vet that discusses some of the risks associated with vaccination. Go to Are Vaccines Safe?