2. Baby doesn't wake all the way up, and therefore I can settle her back to sleep quicker. No waiting until she's all the way awake, then going across the hallway into her room. She doesn't have a chance to get fully awakened and upset this way.
3. I'm lazy. I don't like to get up at night. I already have to do that once or twice to go to the bathroom, so I like limiting my up-and-down to that.
4. I believe, from all I have read about it, that babies were biologically designed to sleep next to their mothers. Infants have irregular breathing paterns, and the mature breahing pattern of the mother in close proximity helps to keep the baby breathing normally, thus reducing the risk of SIDS. Mothers have been shown to be so in tune with their babies when sleeping together that they will awaken just before their babies do, and this in-tuneness also ensures that a mother will typically wake if her baby experiences a stop-breathing episode. See more about this from James McKenna.
5. It helps with bonding. I can stay close to my baby all night, giving her warmth and skin-to-skin contact. A newborn doesn't really get it that she is not part of her mother - they have been inseperable for nine months, and it's going to take a while for baby to figure out that they are actually two separate people! Babies have no object permanence in the early months... no understanding of the fact that just because mom isn't visible, she still exists somewhere.
6. We don't have to deal with this mixing up of days andnights business. Baby learns pretty quickly that it's nighttime because mom and dad are laying in bed sleeping.
7. They won't want to sleep with me when they're 14. My three year old asked to sleep in her own room shortly after she turned two. And who wouldn't melt waking up next to this every day?