By the assumption that God exists, you need to first identity His intention for the making use of prophecies, instead of purely base your conclusion from a human perspective. On the other hand, under the assumption that God doesn't exist, you don't even need to bother with prophecies.
In Christianity, a prophecy is more like a time trick, to get the attention to have a message content confirmed. Roughly speaking, it is for God to confirm His messages to His believers, and believers only. As for non-believers the prerequisite criteria is for them to deliver their faith first before they can get to a confirmation. This is so because by the Covenant one needs faith to be saved. A confirmation comes before the delivery of your faith could simply mean that you cannot be saved.
So the end effect of a prophecy is to confirm with the believers but doesn't need to make any sense to the non-believers.
Matthew 25:29 (NIV) For everyone who has will be given more, and he will have an abundance. Whoever does not have, even what he has will be taken from him.