You know it all depends on the risks you are willing to take, just like vaxing your own kids. If you vax your kids/cats you are taking the risk of the vax plus getting the illness and if you don't then you risk you kids/cats catching whatever they were supposed to be vaxed for.
I personally have 3 cats all indoor/outdoor cats. I hate kitty litter so when they are old enough they are encouraged to go outside to use the potty. My female cats are 10, 3, and 2. After they get fixed and a rabies shot I don't take them to the vet anymore, even to get a rabies booster. They all appear to be healthy and I know they are happy.
My MIL had a cat a couple of years ago that they religiously took to the vet, got all his shots plus extras, spoiled him rotten and only lived 2 years. He got some urinary tract problem, twice. The first time they paid over $1000 for surgery to remove the problem, but after the second time they decided it must be his time to go. He was a indoor/outdoor cat but only stayed in the yard when he was outside. I'm sure there are stories for both shots and no shots like this one.
Getting cats' shots doesn't necessarily mean that they'll live longer, healthier lives from my experience. The last 2 cats didn't even go to the vet while they were kittens. I went to the tractor supply store and gave them their first shots myself. I haven't done any research on vaxes for pets. This is only from my years of living with cats.