I haven't had a chance to read all of the replies, but often when I hear of people speaking of 50% shared care living arrangements, they usually talk of equality for the parents as opposed to what is best for the child. I think for a young child it could be very detrimental, especially a baby/toddler where they need ONE primary caregiver.
I know in our situation, if XDP fought for 50% shared (well, he wouldn't get it but lets pretend) DS's schooling would be interrupted. He wouldn't have a normal home life. Wheras he had a normal home life when he lived primarily with me and had regular visits with his dad.
Also, it's really unfair on very young children that when a relationship ends all of a sudden there is this need for 50/50 - when before they spent the majority of their time with one parent, with regular but short time with the other after work/weekends.
I know my ex fought tooth and nail to get out of parenting when we were together. This isn't uncommon. If he were a decent dad the good news would've been that DS would have had time with his dad, wheras before he was just this dude who slept there and barely interacted with him because he was too 'tired' after work.
I definitely think though where a child is older and the parents live within a reasonable distance from the school it can work, if the child wishes. So long as they are all on the same page re schooling, friendships etc otherwise it could all get very confusing.