One very simple reason is when a site comes under this kind of attack, it can be extremely difficult to determine what traffic is legitimate from what is malicious. Would you want legitimate requests to your site to be 'switched off', if the attack could be mitigated by other means?
I am just wondering why a site under attack can't be switched off so that the rest of the server isn't affected? Why can't such a site then be moved to a different server or something so that any changes being made only affect that one particular site?
One method to assure you have no future problems is to move your account to a dedicated server, then you have completely isolated yourself from any other sites.
Mike, I co-admin a site owned by one of your customers and I stopped by to offer my thanks for handling the attack as efficiently you did. The site was a bit lazy for a few hours, but once propagation had taken place, we bounced right back. I understand the situation you were in and I do not envy you at all. Your actions exemplified your level of dedication to your customers and did not go unnoticed.
For anyone looking for a new host, MDDHosting takes great care of their customers and I suggest you give them a closer look. It's simple to be a great host when things are running smoothly. Mike proved himself and his customer service in a difficult, trying situation and that says a lot.