There is a lot of good information on shared care contained within this article
It's clearly a developmental disaster for children, who under perform at school, use up their emotional resources trying to switch from one home dynamic to another, and cannot fail to be affected by the tension that results from two estranged parents constantly attempting to negotiate arrangements.
For example, boys in shared care are more likely to have hyperactivity and attention problems.
Many of the problems seen by these kids are those seen in foster kids ie attachment disorders and disturbances coming from not having a consistent environment.
Every person deserves a stable, consistent home. I have no idea why this is lost sight of when it comes to children, especially those already dealing with breakup.