I've heard this but I don't buy it. People have been sleeping this way for millions of years. Normally I believe parents are plenty in tune with their baby's presence not to roll over on them. If you're a heavy drinker then yeah you should probably go sleep on the couch. But as a parent you often have to make choices with conflicting information. If you want the best of both worlds get a co-sleeper.
First off mattresses and the way we sleep now are only a few hundred years old. Second we don't have detailed statistics for infant deaths until very recently so we don't know how many babies were dying a long time ago and for what causes.
For evidence of the risk please see my reply below http://news.ycombinator.com/item?id=1839044
When she started climbing out of her crib we put a foam mattress on the floor and let her sleep on that (which we moved to her bedroom). This also helped with comforting her at night when she woke up, as we would lie down next to her. Gradually she stopped waking up at night, and if she did, she would go back to sleep by herself.
IMO, getting your baby to sleep through the night and going to bed without too much fuss pays off. I'm no child psychologist, but I think by assuring them that you're there whenever they need you, they become more secure when you're not around. Being able to have a full night's rest is imperative to functioning well the next day.
Once you have children, you tend to start strategizing your sleep schedule.
Also if you sleep fine, good for you. I wouldn't and couldn't sleep well knowing there's a risk (although small) of hurting a baby.
The risk of SIDS is lower with co-sleeping, and the risk of suffocation or those other horror stories is minuscule. And on top of that they typically only happen with parents who are morbidly obese or on medication (not just illegal drugs, but things like cold medication).
So the prudent thing is not to cosleep it if you took anything, but otherwise it's better.
No. Everybody gets less sleep and for small babies there's a serious risk of death.