It depends on the cat and where you live. We live on a large lot in a gated community with a ravine behind and 25 mph speed limit. We have three cats, all nine years old. Yolanka is from Hungary and was born outside- a rescued ferrel cat. She's smart about cars and is a large healthy cat. She HAS to go outdoors just a bit, and I think it's healthier for them to get their paws on the earth! The other 2 (my late father in law's) we're also rescue, but were declawed. Trixie is a large American longhair, and goes out a bit with Yolanka, 1 or 2 x day but not after dark. Polly is very small and timid so we don't let her out - she has a tendency to wander. I'm thinking of getting a leash or doing a deck set up so at least she can get fresh air. Weather permitting I leave a window open with a stool in front so she can have fresh air and feel a bit of the outside. There are also no other cats in the area and the two stay on the property and come when called (usually!) the 2 cats are too big for hawks, and there are too many trees in out area for eagles.
If you live in a busy or built-up area with any traffic I think it's crazy to let them out unless in a secured area. But they have to have access to fresh air and the sounds of nature.