Isaac was born in extreme old age to a woman with no other children.
Samuel was born to a woman who was unable to have children for many years and prayed continually for a son.
Though I can't answer your usage questions, I can tell you that even today, devout Christian parents who have a son in one of those circumstances very often choose those names. I know several.
This message was edited by the author on June 16, 2011 at 6:20:19 AM