I suppose I dont really have any 'scientific' research for you to look at (well, none that you can't just find yourself, on google) but I can say that I would hate to be left alone in a room with no understanding of when my care giver was returning and if they were returning or why they had left me.
I dont think it makes for strong attachment and there are lots of studies to show that attachment in the first six months is so so important to the rest of life.
Look up John Bowlby Attachment Theory, or the work that Hanslow did shortly afterwards. Also the work at the 'center for child development' in America, looking at secure and avoidant attachment theories.
For me, I dont know whether or not controlled crying causes issues with attachment, but I certainly know that responding to a child as they need it, waiting until it is age appropriate to begin to bring in 'self soothing' etc, certainly does not cause attachment issues.
For me, it was around 10/12 months when my children showed that they had developed an understanding of their surroundings enough to appreciate that when put in the bedroom, they were not there 'for ever' or completely alone.
So from this point we began to slowly help them learn to sleep alone, in our bed as usual to start with, then their own cot but in our room and eventually, at around 18 months, in their own rooms.
Its worked really well and our attachment to our children is just fantastic.
Obviously there are reason why such an approach to parenting cannot be taken, some children require a different approach and some mothers/families need a different approach. In these cases its always worth weighing up the pros and the cons, but really, the ideal should be an ongoing secure attachment for the first six months minimum, with a gentle introduction to independence from that point.