It's just not good for them. And it's too hard for us to listen to when our parenting instincts tell us to stop the crying. I would recommend not trying (or continuing) it.
The littlest kids are not crying for any other reason than to communicate displeasure with something. And it gets our attention. So much so, that I think kids develop a parent- specific pitch that affects us more than any other person listening to that same cry. Smart plan-- it keeps the kids safe and lets them know that their parents are there and meeting their needs.
Most of the time we are trying "crying it out" to get some sleep. You probably want your child to sleep through the night. I don't know what that means most of the time. Most kids don't sleep through the night. We don't either. We wake up every 90 minutes to 2 hours as part of normal sleep cycles. (More on sleep stuff here)
Many of the kids are crying because they are in a separate room and they want some parental contact. Sleep training techniques that suggest that you allow children to cry to learn to soothe themselves have never been shown to do anything good for children. They have been associated with attachment issues, behavior problems and anxiety.
There is a phenomenon called "learned helplessness" that comes from some experiments that showed that if you ask enough and never get what you need, you stop asking. You learn, in the case of a crying baby, not that you are "soothed" but that nobody is coming. That's not something, I imagine, that most people want their kids to learn. And ask yourself-- can you soothe yourself? I'm almost 40 and I'm pretty sure I can't. I see commercial after commercial for Ambien and Lunesta, so I'm pretty sure the adult population has sleep problems. Plus, as an adult, we can get up, read, watch TV, get warm milk, or call a friend. People who don't feel safe in their environment, like those suffering from depression will often say they can't sleep, or stay asleep.
What can our kids do if they can't sleep, if they don't feel safe in their environment? They can't take medication, read, watch TV, or do anything like we can. My advice, if the kids are crying, is to go in and comfort them. Nurse them if you are nursing. Snuggle and enjoy. If you want a family bed, if you want to comfort your crying child, go for it.
I hear all the time "OOOOH don't do that, once they sleep in your bed or you give in, it'll never stop." Sure it will. In fact, there are going to be days when your children are going to be embarrassed to be seen in public with you. Their childhood will be over before you know it. Enjoy your time with them while it lasts.