The purpose of modified meantone was to increase the number of good M3s to nine or more. 
The price paid for this was harmonic waste in the major triads C#, Eb, Bb, F.

• D'Alembert: French mathematician. 

• Britannica: By 1797, tuners had learned to modify the meantone temperament so that nine keys 
  could be used. The Britannica described this temperament as "better adapted than any other 
  to keyed instruments."

• Hawkes: "Improved Modified Meantone" (1807)

• Fisher: A Yale mathematics professor, Fisher studied musical compositions for interval usage, 
  and constructed this temperament so that the C, G, D and A major thirds would be perfect.