It's more extensible because, if you decide you need more enharmonic notes included in the data structure, you can easily add them. For example:
It's also more extensible because it's a hash of a hash. More MIDI attributes can easily be added to the existing data structure as needed (e.g., absolute cents).
(By the way, on the webpage titled MIDI Note Number and Frequency Table, there's at least one error. The frequency of MIDI note number 115, g⁵, is 6271.9269757080, not 5919.9107633862.)