Replying to twilight_guy, not quoting to avoid massive wall of text. Well, the thing is the difference in tax rates could be said to breed discontent. The rich can feel bad about how little tax the poor are paying (yes, i understand this is morally dubious) and the poor can feel bad about how the government will remove much of the wealth from increased wages. With a flat tax rate there's none of that. A proportion of your income will always be taxed at the same rate, regardless of income. Perhaps even social mobility will increase, though i concede this is a tenuous claim.
Oh, and for the record i live in a relatively deprived area, so i'm not a rich person looking down upon the proletariat.