Look, I agree that these devices are mostly unnecessary, but mostly is the key word. If you have an infant that died of SIDS*, you would find absolutely no comfort in the fact that the doctor told you that you'd probably not need this monitor.
Some parents do need to calm down and not rush their kids to the ER for every hangnail, and if your monitor alarms but your baby is fine, then be grateful, but don't freak out. There will be some children saved by this device, and those parents will be more than happy with every penny they spent on it.
My kids are all past the age of needing to worry about this, but I've gone through a few heart-stopping moments with each of them. Honestly, I'd probably buy one, but an alarm wouldn't automatically send me to the hospital (I would call an ambulance, of course, if my child were unconscious and not breathing, and I have had to do that once).
*Doctors say they don't know what causes SIDS, but I am almost positive that some (incredibly rare, very small percentage) children contort themselves into a position while sleeping that suffocates them. Not all of their biological processes are fully developed and their muscle tone is very poor, so if they get into a position where they can't breathe, there's a good chance they'll suffocate to death. And because everybody is asleep and the whole thing is silent, parents won't know until morning. This monitor would cure SIDS if my theory is right.