This is another thought.. Pillar of Autumn didn't leave as soon as the Covenant invaded Reach. It would've taken a while to get Cortana to the Pillar Of Autumn with the Covenant trying to stop their progress. With Cortana being the 'last hope', my thoughts is that IF (not saying it happens cause it doesn't) Halsey were to have died, all her knowledge in everything (which is a lot and important things) would not be lost because it is in Cortana.