While growing up I was told, its okay to let a child "cry it out" and that if you run to them every time they cry, you are spoiling them. This is not true. If you let a child cry it out before they are 1 year old, it could cause brain damage. Besides this if you are not responding to your childs cry, you are not building trust. Trust leads to attachment.
You want to encourage attachment. Its important that the child attaches to the main care giver or else some other adult. If a child grows up without it, its proven to cause some disorders and/or difficulty getting along in the social environment. Responding to your child's cry is building trust. If you are not building trust, you are not going to reach attachment. There has been studies in the field. If you have any questions, I would be more than happy to point you towards some articles with more information.