that was just to show that akatosh may have never broken the link.
Also why would akatosh give someone the soul of a dragon if their only task was to light fires. Why have a dragonborn kill his own children. It seems likely that the dragonborn is meant to be a leader of some kind. Septeim was an emperor, and as stated by paarthunrax all with the soul of a dragon have an urge for power.
That's what I was thinking. I originally thought the Dragonborn would become emperor because of his ability to light the dragonfires, but then I remembered that they no longer exist.
Maybe the dragons are Akatoshs last link to Mundus. Maybe that's why he chose the Dovahkiin to slay Alduin. Maybe that's why the blades, an order essentially dedicated to him, want you to kill Paarthurnax. Akatosh knew Alduin was returning, knew that he would raise the other dragons. Maybe he let it happen so the Dovahkiin could destroy their souls. Maybe Akatosh is tieng up loose ends, and you'll be the last one. :l