I just read Attachment Parenting by Katie Granju and it sounded a lot like what Dr. Sears says in The Baby Book. But one item was particularly interesting. Granju makes the distinction between NURSING a baby and BREASTFEEDING a baby.
She says that breastfeeding is the transfer of calories from and nutrition from mom to baby. But NURSING describes a style of parenting through breastfeeding. In other words, moms who are nursing are using breastfeeding as a tool in their relationship with their baby. What some call non-nutritive sucking is, according to Granju and her cited experts, fulfilling baby's physical and psychological need to suck. In addition to filling their little tummies, nursing calms, soothes, relaxes, and shows baby love.