What are your thoughts on allowing cats outdoors?

I know this is generally felt to be a bad idea for the cat’s longevity and non-awful demise, but a good idea for the cat’s general happiness and weight. I’ve walked my cat on a leash and harness for a while now, but I recently moved to a rural area – not far from the Magnificent Mr. Milo, who is an indoor/outdoor kitty. Maybe a dozen cars drive by per day, and my house is set back from the road. I found myself pondering the other day about picking up one of the (fixed, vaccinated) barn cats people offer free on craigslist to rid my shed of rodents, and then realized many people would think me silly for not just letting my own cat outside to do the dirty work.

The thing that makes me hesitate is that there’s predators, other cats with diseases, those dozen cars, songbirds, antifreeze leaking from abandoned machinery, etc. out there for him to kill himself with. I mean, say he was a small dog – would I let him roam around, knowing he’d potentially get into this stuff, in lieu of taking him for walks? I think not. But…dogs seem satisfied with walks in a way cats don’t. My cat doesn’t get to dustbathe or hunt stalks of grass or really even be very active with the harness on, since I balk at going into the numerous thickets he wants to explore. My fiance thinks I’m crazy to want to keep him an indoor-only cat out here, but then he grew up with outdoor cats and I grew up with indoor cats.

I think what I need here is more arguments both for and against, and related experiences about outdoor cats. Would you guys care to share some?

Also, what is it like to have an outdoor cat? Can you train them to come when you call, if they are within hearing range? Do you worry about them if they don’t come back on a regular schedule? I’ve heard some are big wanderers. Have you had an outdoor cat who lived to a ripe old age?