As a child of divorce being dragged about from home to home having to split your life is never in the best interest of the child. Thats why its virtually never done, its near impossible to have a 50/50 split that doesn't negatively impact a child.
Would you want to be passed back and forth between two homes, families (step parents and siblings), their different sets of rules etc... every 3 to four days? What about school, friendships, your belonging, groups, the vibe of the environment and just the right to ever relax and feel at home in your own bedroom.
As for 'a relationship' with the other parent, thats up to the CHILD not the adult. I can safely say I did NOT want to live with my dad or be moved around and thats what the court agreed with, it was MY choice.
My father did get visitation which he fought for because he thought it would get him out of paying (not that he ever did pay anyway) and then he never showed up to it. Parents that view custody as a way to 'not pay' are never doing it in the interest of 'whats best for the kids'.
The idea that all kids have perfect parents that they love 50/50 and that they benefit from both being equally in their life is a fantasy land. Millions of us have had shitty, deadbeat, abusive parents that we where far better off without.
Even if both parents are loving and kind the kid doesn't benefit from being dragged up, they a better off in one home and just seeing the other and parent that care for the kid over themselves wouldn't insist on fighting over them just to suit what they 'want' to be seen as.