I agree, withholding an address can clearly impact on child's welfare.
Obvious practical example - when I was having chemo, strong chance over six months I would need urgent hospitalisation at short notice. Once my ex went to stay in relatives house over weekend, refused to give address or landline, saying I could call his mobile. Only problem was no reception at relatives house. Why on earth, if there is an emergency should there be any difficulty at all about contacting the parent who has the child. Why should my parents have to rush about ringing his sister or mother, trying to track him down. What would have been the impact on my daughter if I had been very unwell or even died and she found out at dropping off time?
You may think this is extreme or unlikely example, to which my response is - tough shit. I was primary carer, I know full well I was more responsible and pro active than my ex. I did not fear he would deliberately hurt her but I knew from previous experience that he wouldn't be too fussed if she was hungry or tired. So I did worry when she was little - now she is 8, I worry less. But if he wants to play silly games with me, I draw the line very hard and fast. I have all the stress and strains of being the main carer so he will respect that, even if he obviously neither likes nor respects me as a person.
Ad I think such disrespect for the child's other parent is harmful to the child in other ways. It causes distress to the parent who worries aout where his or her child is, that distress may well impact on parents emotional availability to the child.
It is just a bloody stupid, needless, provocative piece of nastiness, to refuse to reassure the other parent. My ex has done it to me so I have experienced this both as a lawyer and as a mother.
The stress it caused me was such that I took a hard line and simply said - you will not take our daughter out of my care overnight unless I know where she is. If he had wanted to take me to court over this I was quite comfortable as I know well how the courts would view his behaviour.