Why do you assume he refuses? Maybe his will is provoked and he repents this time?
Praying could make the same difference that Moses' prayer for the Israelites made. God withheld his wrath and was patient with them, showing them mercy.
Yes, sometimes a man refuses, sometimes he doesn't...the point is that it is the person's choice. God has given him all he needs to come, but he must be willing. With us, prayer has the potential to bring change, but in your system it matter's not...it has already been determined.
If God can't override man's will, then praying is of no value. Period.