I know of no situation in which a baby has gained any advantage from having a pacifier inserted in his or her mouth. I believe it fair to say that the only people to gain from the use of a dummy are the child's parents or caregivers--hence the term pacifier itself. Furthermore, in 40 years of dentistry, I have seen literally hundreds of young mouths severely damaged by dental decay and by dento-facial distortion, very probably as a direct result of the excessive use and abuse of these devices.
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Thumb-sucking has a major advantage over a pacifier, in that when the child enters an episode of activity, playing or moving about, the thumb will invariably be removed from the mouth; a pacifier tends to remain in position. So the potential for dento-facial distortion is perhaps less with the thumb, at least in a child of normal activity.