Flexibility. You can turn crude into any of a number of petroleum products as needed, but once it's refined, you've locked yourself into one product. If it were a heating oil shortage, and our reserves had all already been refined to gasoline, we'd be up a creek, freezing our butts off.
The problem is the same as what led to NO being flooded. Inadequate risk analysis being performed on our infrastructure. Yes, having refining capacity at the mouth of the Mississippi river allows us to distribute the gasoline all throughout the middle United States, but putting too large a percentage of our refining capacity in one geographic area leaves us vulnerable to a natural disaster.