I'd be lying if I said I hadn't thought about it myself, especially when the child's safety is in question and the proper authorities won't listen, but common sense always prevailed. The state takes kidnapping by non-custodial parents VERY seriously. If caught, the parent is likely to end up in prison, and likely to have few or no visitation rights when he or she gets out. That says that it's never justified because, inevitably, things don't turn out well -- and the main person who suffers is the child.
As for if it should be prosecuted to the fullest extent of the law...that's always a tough call. Sometimes it IS done in the child's best interests, and I suspect some of the tough sentences are to deter other non-custodial parents from thinking about it. How would someone prove one way or another? Sadly, it seems the only avenue is to keep pushing local authorities and gathering evidence on the potentially dangerous custodial parent until something happens through the proper channels. Child protective services, unfortunately, have a habit of not listening to a non-custodial parent, but it's the option that's available.
So -- is it ever justified? Undoubtedly. Is it the best route? No, even if others aren't working. Does it work out for the best? In 99.999% of the cases, no. Most end with the non-custodial parent losing the child and getting prison time, so if the custodial parent truly is dangerous then the child just lost his or her greatest protector and advocate.