My theory of letting the kids sleep on their own is - it gives the mom and dad a chance be sane parents.
Research says that it helps the child to be a better child and succeed in life.... i am not sure about that :). Children over many generations have been rocked by parents or in the cradle, i am sure those children have been very much successfull.
I might agree on the point that it does help find the security they need in themselves and also its the path towards emotional independence. Emotional independence is something that can be debated on and i think its a topic in itself to write about.
Siddhant has been sleeping on his own for the past year now, and i am not sure whether I see much of the emotional independence in him that he is supposed to have. My hope is that, this sets the precedence towards that path.