If your cats have your house as their territory, they are more bonded to you and make better, more loving pets. Cats frequently get stolen and many are sold to laboratories for experimental use. Cats get hit by cars, injured by other animals, tortured by sickos, lost, poisoned, locked in sheds or garages. If this happens, you will likely lose your kitty to a horrible viral disease even if you have had her vaccinated. Your cat will probably bring home fleas.
Cats get tapeworm from fleas.
Once your home is infested with these pests, their eggs can take up to two years to hatch. Your cat can get many other parasites and illnesses from contact with outdoor animals, including ringworm which is a fungal skin infection that humans can get. You don't let your children play on the highway. If you take your cat out on a harness and leash, you are not protecting her and not doing her a favor. Bye bye kitty.