For them to be valuable as testimony to an historical event they would have to be eyewitness records of events and would have to be subjected to examination as such.

If they were the written records of oral recounting of events that happened several generations before they would lose a great deal of value as testimony.

But then this is a matter of faith and it doesn't matter to Christians about the evidence value of the Gospels, I would think it's the spirit of the Gospels that matter to believers.