Quoting Minion:" This stems off a Facebook post I saw today. It read: "I don't think I will ever be able to do the ... [snip!] ... he/she is crying, especially when they're calling for you. Jo is one of the happiest kids I know and she has never CIO before."
Every baby is different and every stuation is unique. MOST babies learn to self-soothe and get themselves to sleep by the time they are 6 months. SOME dont. At some point, if the baby isnt getting the sleep it needs, it is in the baby's best interest to CIO. But, it's for when all else fails.