Why we started rice cereal when we did
Mostly, because our pediatrician suggested it.
At the four month appointment, our doctor said that we could try to start giving our baby rice cereal and early fruits. We had reservations about rice as there was recently an article about arsenic amounts in baby cereals, and she said that she read the study and didn't find the amounts concerning enough to not recommend it. She liked exposing breast-fed babies to cereal early because most won't give up breastfeeding, and rice and early foods will just supplement and expose baby to a different diet.
We liked the idea of letting our daughter eat more than just breast milk because I was pumping at work, and I had a pretty much break-even supply--I made just enough milk to cover my absence, and I had a very small cushion. It was helpful to know that if what I pumped didn't cover my daughter's hunger, she could still be satiated by rice cereal, and then I could nurse again when I got home.
Did she eat much cereal? Only in those two months--once we started introducing more foods around six months, she stopped eating it entirely.