While they are more likely to pick up viruses from other cats and parasites from animals they eat, it is really their efficiency in killing songbirds and other small creatures that led to our decision. No need for a tag inside the house.

A long time ago we had a cat that we let outside. She almost got killed sitting in the neighbor's car engine, and once got bitten by some animal and got very sick. Besides that, ticks, fleas, ear mites and other parasites can get to them, all costing money to fix. Our new kitty was a stray, had ear mites and we've spent over $200 fixing it. And he's got bald spots from scratching that will take some time to grow back. What a mess.

And in our area there are wild predators that I'd worry about: coyotes, hawks. And getting lost in the woods, or hit by a car. And on top of that, killing songbirds that we attract to our yard with feeders.

Last but not least, they are always and forever wanting "in" and "out". Always on the wrong side of the door! They have a screened porch and a whole house with toys and cat furniture and that should be good enough.

Unless they're "working cats" on a farm
But once they've had a taste of the outdoors it's way too easy for a cat to skip out the door, so microchips or collars are a great idea for those with an escape-prove kitty.
Best advice is to never let then out ever! Unfortunately my grandcat Eddie was allowed "just on the deck" and now slips out the back door at the slightest opportunity. He has a collar with an id tag and they're doing the squirt gun thing to train him away from the door.