I have a theory that baby monitors are one of the reasons babies have poor sleep habits. I have noticed that parents respond really quickly to noises that they hear on the monitor. The noises are louder through the monitor and I think, if left alone the baby would go back to sleep. I never used them at all. Reckoned that baby needed attention when you could hear crying from another room. My babies were in same room till about a year old. They would sleep morning and afternoon in cot, with me periodically checking.
My youngest is now 11 years old and monitors weren't used as often as they are now. When friends visited with their baby I was amazed that the monitor was on for every sleep and they responded whenever there was a grunt. Any thoughts?