I think it is the way human kind survived in the past. Little ones were sleeping with or next to their parents to be safe and protected. Throughtout the history parents slept with their children, only the richest people didn't but often they hired women called wet nurses who not only breastfed but usually were required to co-sleep with a baby.
Honestly, if we didn't co-sleep I wouldn't be able to lift my toddler's head when she was asleep and choking on her vomit. To me, co-sleeping, done in a safe way, was a valuable bonding tool. It also gave me, a mother, more sleep and certainty that my babies are safe, sound so I could easily relax and fall back to sleep instead of listening to the sounds coming from their bedroom. To my children it was being where they felt loved and secure, next to their mama. Co-sleeping seems to be a natural instinct of any mother, as long as she is not convinced by the society about the dangers of it.
The only real dangers are if parents are drunk or drugged or smoking or seriously ill. Or if you put your baby in a bed without a bedrail. We put our mattress on the floor.