This is the 'there are no atheists in foxholes' argument.
Firstly, even if true (and there's no good reason to suppose that it is, not least because how the hell do you find out from all the now dead atheists whether they prayed before their impending deaths), what does it prove? Is it simply a numbers game? God (see, I used it there, and I don't believe) help you if Islam ever gets more followers, as Allah will suddenly become real...
Moreover, if the line is supposed to indicate that under all this evil, demon-loving atheism, there is actually a true believer waiting to surface, it should be rather worrying that all these people who said, 'Dear God, don't let me die!' did, in fact die. This is in contravention of Matthew 17:20, where Jesus says that if you have true faith, all you have to do is ask, and you will be given. Also cf Matthew 21:21.
The fact that some poeple believe this argument is a good one actually shows is how closely religion is associated with the exploitation of our deepest fears. 'Get someone scared enough - they'll believe.' Nice.