I myself don't really let my cat out... Only when we used to go to country on summer, we took him with us. But in the city he's always at home because there seem too many dangerous things out there (cars, dregs, dogs, doors, etc...). Besides, we live on the 8th floor, and the front door is mostly closed, so it might become tough for him to get back...
Same goes for me, I don't let them out because I live in a 3rd floor apartment and I am terrified for them to start getting "courageous" and falling off the balcony and terrace. I do let them out when I am outside though.
It's not that I don't trust my cats, I know they can balance perfectly and they've got good instincts but... I don't trust my cats Emotion: stick out tongue

Though seriously, I would love for them to experience the outside a lot more so I'm hoping one day I will have a place with at least a small garden for them to enjoy. It would have to be a secluded garden because otherwise I'd be too scared they'd run away... but let's get the garden first hehe.
Nope, mine are totally indoor babies. There have been a very few times when we've been out on the back deck and have invited them to come out with us, but they don't have any idea about survival outside - dogs, foxes, cars, etc. 
That's why when Mickey managed to slip out when it was so freezing cold out and was gone overnight, I was pretty sure I'd never see him again. Thankfully, I was wrong!
Well i dont have cats, but i wouldnt let it out if i had one.
we live on 40 acres of land in the country, but they still go missing. just 2 weeks ago our 3 yr. old fixed female siamese never came home. they go out in the morning and i always get them in at nite....oh well..my baby is gone....i would rather find her dead than the misery of not knowing.........
I let him out, then in, then out, then in, then out, then in....and on and on......
I live in a small town and have a large yard. I have one cat that always stays indoors by choice; several others go in and out several times during the day (I don't have to use much kitty litter). I would prefer that they all stay inside, but they get cabin fever and look for ways to escape. I leave a convenient window cracked for them; some also come in at night when I call them from the front door.

The reason I would like them inside all the time is that there are feral cats all around my house (I feed them, poor things!) and they often have feline HIV or feline leukemia, which are communicable, especially if they fight. In a larger city, I would worry about cars hitting them or their annoying people who would harm them. Some cities will pick up wandering cats and take them to a shelter (where they can eventually be euthanized, even if healthy).

The indoor cats stay close to my house and come inside regularly, especially on cold nights. I have to treat them for fleas on a monthly basis. If you go away for an extended time (more than 3-4 days) they will wander off. I leave dry food and water for them in my porch while I'm away and all of them show up again within 24 hours of my being back at the house.

Yup, our cat Snake can go in and out at will, even at night. She hunts squirrels, birds and mice, and chases away trespassing cats.

My cat lives out at our rented acredge, in a shed with her kittens for a 'house', and is a great mouser! (And birder--on top of that she eats everything she catches, or gives it to the kittens). She likes going inside our cabin, but ultimately she's out 98% of the week. We aren't even around that much sometimes. So far, no injuries except for a scratched nose and she's gotten ear mites twice, but she loves patrolling her territory and hunting that the benifits far outway the risks.