I have both indoor and outdoor sleepers. For the most part, I round the indoor/outdoor kitties in for the night. There are two cats, however that never come insides; barn cats. They can't live in the house due to urination problems or the fact that they hate other cats. But they both sleep in the barn, where they have cat beds.
It is safer for the cat to be indoors because large predators come out at night: foxes, racoons, coyotes, wolves (in certain areas), mountain lions, and bears. Now bears and wolves are less likely to eat your cats, but the rest of the listed predators are usually the culprits.
Domestic cats are just that, domestic. They were bred for companionship and showy colors. They retain a lot of their predator wild instincts, as we all do, but for the most part it is a shallow base of instincts. I'm sure you've experienced where your indoor/outdoor kitty will kill a mouse or almost kill a lizard and do nothing but play with it. Never eat it. The instinct is to catch something moving fast. Not to necesarily kill it properly or eat it.
By expecting domestic cats to respond to their environment like a savvy wild cat, you are giving them more credit than their breeding and in most times, their upbringing. Cats raised in the home and let out with older cats once in a while until they're old enough is not a full-on boot camp into surviving outside like they might have gotten where they feral or wild.
There are some cats that just can't be inside. Ferals, for instance. And in different cases, random personality types. But most animal rescuers agree, if you are going to take the danger of letting your animal outside, you should at least bring them in before dusk when the predators wake up to hunt. And fat cats shouldn't be outside long anyway. ;)
Most cats raised in an indoor-only environment never are even curious about the outdoors. They may want to sit in the sun of a window or go out onto a screened-in back porch and catch flies. But they never know the difference; they live healthy and fullfilling lives indoors. Therefore, it isn't cruel to expect them to live indoors all the time.
Cats brought up outside or in an indoor/outdoor environment tend to be harder to adjust to the indoor only scene. But at the rescue, we do it every day. And most "converted" cats end up living happy lives indoors after the brief awkward stage where they try for the door whenever you open it.
So, most of my cats (and most of the cats I know) are in before dusk and let out after dawn for safety reasons. A lot of people have differing ideas, but what it boils down to is that you are the animal's owner and protector. You are the one responsible for the animal's safety. It's better to be safe than sorry. For their sake more than your emotional attachment. Indoor at night! =)
Animal rescuer of over 3 years and long time cat owner ;)