they still were not protecting Sephiroth... when they were released from the Northern Crater, it's because the crater is where they were originaly frozen/sleeping, they were releasd back into the world... because they were trying to wipe out all threats to Gaia i'm certain they would have gone back to the crater to take out Sephiroth & GENOVA's remains too if they could only get through that barrier when they were done taking out all the Mako Reactors...