But we are wasting the valuable Fusion time on it. More than this, Elesa isn't willing to wait for us forever. If we run of steam, we won't go back to safety to recharge: we will remain in the dungeon. At some point mullish stubbornness stops being attractive character trait, and I don't want our current benefactor/leash holder to point it out to Shinji. Do you?
We get the full power and reliability of Fusion back if we accept the (temporary!) seal though. Why choose to gamble?