It not about the sound of the clashes of b9's its just about rules, and thats what the rules state.
have a look at clinton roemer's classic text "the art of music copying" or sammy nestico book "the complete arranger".
Both general regarded as the authortive methods of western music theory and refernced by most professional arrangers / composers.
They both state quite cleary that the spelling of A11 necassary does imply the use of a major traid on the bottom.
Im sure this rule is echoed thoughout numerous text's on western harmony,
these are the rules, and im a firm believer of adhereing to them.
sorry if i sound like a smartarse