It would have completely destroyed the vaults, and everything inside.
The firemen had planned to let the roof burn - it would have been impossible to stop, and would have pulled resources from the more inport tasks, namely protecting the towers and keeping the inside temperatures below flash point.
The North Tower was particularly important: the bells are mounted on a wooden frame that is independent of the stonework - when they sound, the wooden structure absorbs the movement and weight changes, so the tower structure isn't stressed. If this structure burnt, the bells would have fallen, causing catastrophic damage to the tower and probably the structure itself.
As it stands, there were only three holes punched through the vault - which is incredible considering the weight and heat directly above them.
The firemen came under intense criticism during and immediately after the blaze, but the proof is most definitely in the result.
