I think it really varies from baby to baby. Sleep training doesn't necessarily involve crying. There are three basic ways to sleep train: extinction (basically, you leave Baby after your pre-sleep routine without going in to him at all), graduated extinction (after your pre-sleep ritual, you leave baby for a few minutes at first, then go soothe for a few minutes, then gradually make the times away longer over a period of several nights), and lastly, there's the check and console technique (after pre-sleep routine, lay Baby in bed while he's drowsy but still awake; if he starts to whimper, soothe him without picking him up; if he cries full out, do whatever is necessary to soothe him, then put him back in bed and try again).
I'm the daughter of a mothering mag. parent (she picked up her first copy of Mothering the day she got out of the hospital with me, and says it's the best thing that ever happened to her as a parent), so I grew up with an 'alternative' style of parenting. I self-weaned at age 5 (I remember it vividly), refused solids until age 2, co-slept until age 2 (and often ended up in bed with my parents, even after I had my own room), was carried on demand until age 4, apart from my first DTP (from which I had an extreme allergic reaction) have never been vaccinated, was homeschooled through grade 12, was brought up Waldorf, was raised vegan...you get it. I'm the older sister of five, older cousin of many (most of whom I've babysat regularly since they were infants), godparent of one, and have been in childcare (parent's helper, babysitter, and nanny) for the past dozen years. I've worked with many, many babies. In my experience, when it comes to sleep, rule #1
is: Everyone in the household needs sleep, and the right amount of it. Rule #2
is: Every child is different, and the how and where she falls asleep should suit the needs of her individual temperament, as well as the needs of the family as a whole.
I used to be a firm believer in not letting a baby cry. Ever. Or if the baby was crying, the parent or caretaker should do everything in their power to comfort them, and by no means leave them to cry alone. I hated working for parents who wanted me to use cry-it-out, but I did, because it was my job. Then, I learned a powerful lesson in parenting (even though I'm not a parent, I still, in a way, parent the children in my care), which is, never make ultimatums.
A couple of years ago, I started working with an infant whose parents were going the attachment, co-sleeping route (which made me very happy). Baby had a co-sleeper, no cot, no cradle, no bassinet. He slept very well in his co-sleeper, both at night, and during the day. I'd put him down for a nap after about 90 minutes of wakefulness, and within 10 minutes of soothing (bottle, rocking, singing), he'd fall asleep and stay asleep for awhile. When I was with him, sometimes he'd sleep 4 hours alone in his little co-sleeper. Sometimes, he'd sleep in the ergo while we took a walk. He was happy, alert, everything was going well. Then, he turned 4 months (I've since learned that this is a crucial age when it comes to sleep patterns and the changing of those patterns). His sleep started getting worse and worse. At night, his parents told me, he'd wake every hour, not always to nurse, and would take awhile to fall back asleep each time. During the day, our 'nap' routine basically became: soothe for 30+ minutes (swaddling, rocking, running the water, white noise, dark room, sling, walks, lullabies - you name it), he'd finally fall asleep, he'd sleep in my arms (he wouldn't sleep in his bed at all) for 10-30 minutes, then he'd wake up screaming, and we'd go through it all again. I knew he was exhausted. We all read "The No-Cry Sleep Solution" and applied the techniques. Baby was happy with his new lovey, happy in his bed (while awake), we used sleep words, waiting by the bed to soothe him before he started to fully wake up...everything. No joy. After two months of things getting worse and worse (and Baby getting crabbier and crabbier), we all finally decided to give extinction training a try. The first night, the parents were expecting the worst. In reality, Baby cried about ten minutes, fell asleep, and slept for 12 hours straight. Naps took slightly longer for him to get used to, but there were daily improvements. I knew he was crying less total with extinction sleep training than before, when I was there, trying to soothe him, and he was definitely sleeping more (I kept copious notes).
It's been over two years since that experience, and Baby is still a good sleeper. He's attentive, alert, and happy. He trusts his parents and myself. He feels safe in his home, and he looks forward to his nap and bedtime (sometimes he puts me to 'sleep', reading me a story, giving me a bottle, singing me a lullaby - bless him!). I've also worked with a lot of babies whose parents chose not to make sleep a priority. I'm sure there are other factors, but from my observation, those babies consistently have more trouble taking in new information, are quicker to lose control (burst into tears so quickly that they don't even have time to explain to me what they're upset about; and will do this multiple times within the space of a few hours), have more trouble focusing, and all around seem to be displaying several signs of exhaustion.
So, bottom line, I recognise (although I can't relate to the same degree) a mother's agony at the mere thought of leaving her baby to 'cry-it-out'. And sometimes a baby will benefit much more from co-sleeping or some form of no-cry sleep training. Sometimes, extinction or graduated extinction training is so stressful on the child that it makes the child more overtired, rather than helping her to sleep better. But sometimes, a baby really just needs to be left alone. Either way, if the parents and/or the child aren't getting enough sleep, something has to change. Sometimes that change means extinction training. Sometimes that change means co-sleeping. But whatever you choose to do, parents need sleep and children need sleep. If the parents aren't well rested, they cannot be good parents (or if they are getting by, they will be even better parents when they aren't exhausted). If the children aren't well rested, they won't develop as well, they will struggle to learn new things, they will be more unhappy during waking hours, and they will be more susceptible to illness and injury. Sleep is important. Very important. It doesn't matter whether you're co-sleeping or are all in separate rooms. It doesn't matter if you have a 2-hour pre-sleep ritual or if you have a simple, 15 minutes - goodnight routine. What does matter is that everyone's sleeping, and everyone's getting the right amount of sleep for the age and life-stage they're currently at.