As an addendum to that, extra grooming won't take care of all parasites that can be picked up by pet cats allowed outdoors. If they eat prey infested with worms, they too will become infested and the worms will drop from their butts -- all over the house! (This is one of the less pleasant memories of my childhood!) So one incurs the added expense of worm medication, and perhaps extra trips to the vet.
My parents let their pet cats go in and out, and I went through the experience of losing several childhood pets to disappearance as well as an early death presumably from a car collision, and dealing with their injuries from fights with neighbors' cats. One cat disappeared for several days and came home with a broken hip (we never learned why -- kicked by a horse? Abused by teens?) which required months of recuperation and left him unable to spread his legs when washing himself. Then there were the female cats my parents refused to spray (though they had their male cats neutered), so I watched many, many litters of kittens get taken away to the SPCA where I'm sure most were euthanized. There are so many reasons to keep kitty inside besides the prey-preservation factor.
There are sacrifices to be made if one keeps a cat indoors 24/7: one must either clip the cat's claws, take the drastic step of declawing it (not recommended!), or resign oneself to having to take the cat into consideration when decorating -- getting vertical blinds instead of curtains, adding scratching posts to the feng shui (and training your cat to use them!) and treating sofas with repellant lest they become scratching posts also. To me, it's worth working around the cat's needs and keeping her indoors. Fortunately, she loves destroying her scratching posts and, for the most part, leaves the people-furniture alone!