A friend of mine in college had a loft in her room, and I was surprised it didn't have a bed rail or anything (although, college lofts usually don't and bunk beds at summer camps usually don't). I asked her if she was afraid she would fall out at night. She said, "Why, are you afraid you're going to fall out of your bed at night?" I thought, good point. The only reason it concerned me was because of the height.
Co-sleeping is extremely safe if there are no alcohol or drugs involved. The biggest fear people have is that they would role over on top of their baby. Why would you do that? Do your routinely role over on top of your SO or your dog or cat? Both of our children co-slept with us, and I can honestly say the idea of rolling over on top of them is ludicrous. It won't happen. People are able to get a good night sleep and still be aware of the baby sleeping next to them.
You asked why people co-sleep when they could just have sleeping arrangements similar to yours. We co-slept because there were many, many, many nights when that was the only way our kids would sleep. Our kids did not want to sleep alone, they wanted to sleep next to us. Our kids began almost every night in their own beds. If they wake up in the middle of the night and have trouble going back to sleep on their own, they come into our room, crawl into our bed and go right back to sleep. We found out very early on that if we tried to get them to go back to sleep in their own bed, it would involve actual work. No thank you. We just brought them to bed with us, and everyone was happy.
A good friend of mine put everything into perspective for me when she said, "Who cares where they sleep as long as they sleep, because then you get to sleep."