I think noonki's post sums up the problems very well. First, there's no ethical way of determining all of this. The studies done (on those poor Romanian children) only show that prolonged periods of crying and lack of response are damaging to children.
Second, for some parents there is no other tolerable alternative - they simply can't manage unless they do it. And this is a consequence of modern life: we're so isolated, living, for the most part, in couples, that if we can't cope, then that's it: what else can you do? In our past, I imagine, large family groups would have been around to take the baby, rock it, whatever, when mum had had enough. Certainly, in prehistorical times, it would have been dangerous, I imagine, to leave a baby crying on its own (even if that were possible - people would've lived in one-room/cave environments).
I, for one, don't believe that always rocking a baby to sleep prevents it learning how to sleep on its own. In native cultures around the world, babies are rarely left on their own to sleep, and tend always to be in a sling with mum. I don't see millions of Africans or Asians struggling to sleep at night...