This interesting article from the Sacramento Bee notes that anti-Pledge of Allegiance litigant Michael Newdow and the American Law Institute both are arguing against the continued application of the "best interests of the child" test.
The article says that ALI "recommends substituting an 'approximation' test. Custody would reflect the approximate actual division of parental responsibility before a family splits apart." One expert cited in the article "suggests custody be awarded by a coin toss because 'there usually is no rational basis for preferring one parent over another.' He argues flipping a coin would be preferable to having judges evaluate parents' intrinsic worth, as under the current system. He says it also would give both parents an equal chance and reduce the harm that constant litigation does to children."
I myself have litigated exactly one (temporary) custody matter, and lost, and what a terrible day that was. The dispute involved what might be a common scenario - one side had money, the other side had a lifelong bond with the child. The judge, a good and fair man, sided with the money. His ruling did not make much difference in the end, the parties "worked things out" after the hearing, as there was no animosity between them, only a very genuine disagreement about what was in "the best interests of the child."