What is the justification for relying on faith healing over conventional medicine?

Every so often, I read about Christian parents choosing faith healing over conventional medicine for their sick children, in some cases with lethal results. This usually results in the parents being convicted of either severe neglect or manslaughter. (Rightfully so, in my opinion) I know there are several accounts of miraculous healing mentioned in the Bible, and I don't see a problem with prayer _in addition_ to medicine, but **what is the justification for opting to rely on prayer alone?** Is the answer different when it's an adult making the choice for themselves rather than for their child?