That is the earthquake scale....
that's what I thought, but Bastardi was explaining the same thing yesterday.
The moment magnitude scale for earthquakes is based on energy, not damage. The Saffir-Simpson hurricane scale (and I believe I am correct) is for the amount of damage. It was invented because prior to its implementation there was no way to accurately estimate the devastation caused by a hurricane. The damage is listed by the typical description of damage (i.e weak, moderate, extensive, extreme, and catastrophic). The rule of 10 is just a rule of thumb. The damage any hurricane causes depends on several factors including the location it hits, the tide, the wind speeds, its size, etc. I tend to believe the rule of 10 is fairly accurate though.