I don't know where the dividing line is but I wouldn't worry about the 30s. Dogs are different, but my sister keeps her dog outside year round, and only takes him in if it's below 0. It probably depends, too, if the outdoor cat has a little nest she hangs out in.
I have an indoor/outdoor cat, and she goes out in all weather. When it's cold she does tend to run inside.
(And my opinion on the great indoor/outdoor cat debate is that some cats would really rather be outdoors. The risk is worth it for the increased happiness that some cats get from being outdoors. I'm sure birds in cages are "safe" too.)