There are arguably two competing directives, one is too get the most revenue per disutility generated, in which case the flat tax is definitely not desirable. But another is fairness, it seems unfair to only make certain people suffer disutility, this logic is commonly accepted when discussing minority rights. So maybe the flat tax is minority rights for the rich? And what societies try to find a middle ground between the two goals?
Arguably far more concrete notions of utility (such as benefits obtained, or resources consumed, and development of human potential) are preferable for basing policy decisions on, rather than such abstract notions as logarithmic utility of made-up concepts.
