Pacifiers? When? Why?

Baby is grumpy today and out of husband's mouth comes the word 'pacifier'. She is 4 1/2 months and so far didn't have a pacifier. She isn't sucking much on her thumb either. I'm not really comfortable with the idea of a pacifier, but before I say, 'no never not my baby', I'd like some input of other parents: When get babies usually started on pacifiers? What are the benefits? What are the drawbacks?