In my early days of breastfeeding, I had read an article entitled "The Case for the Virgin Gut", which talked about how even giving your baby 1 bottle of formula has risks. I was not really a *purist* since I mixed fed Naima for about 1 month, after she was diagnosed with breastmilk jaundice.
Recent news show that there is basis for this theory of the virgin gut. Scientists have identified an ingredient found in breast milk called pancreatic secretory trypsin inhibitor which lines and protects the babies' intestine. This ingredient is not found in formula milk. Read more about this here.
Considering that it is at its highest levels in colostrum, there is more reason to put babies to breast in the first few days, even if mom feels that her milk has not come in. The list of breastfeeding benefits just gets longer and longer! :)