what is important here is the idea of discretionary income
Gross income - taxes - necessities = Discretionary income
removing tax from the equation first, its pretty clear to see the difference between a poor person and a rich person. most of the poor persons gross income (if not all) is swallowed by the necessities portion of the equation, leaving very little to no income left. the current tax system (or similar variations of it) attempts to attack what is left from gross income after necessities is taken into consideration. the flat tax system, on the other hand, attacks the gross income part of the equation, and doesnt take into consideration the necessities expense. so although in a flat tax system, everyone is technically paying the same percentage on their gross income, when you include a fixed value for necessities into the equation then poor people are paying a much higher portion of their discretionary income towards taxes, which hardly seems fair at all
the fair tax is not a good solution in my opinion either. personally, i love how huckabee calls income tax "productivity tax", trying to imply that we are currently taxing Americans on how productive they are in society, since the more you make the more youre taxed (how unamerican is that...) . anyway, he wants fair tax, or what he calls "consumption tax" which taxes everyone about 25% on all their purchases. my point here is similar to what onassar said, by making sales tax that excessive youre making spending money very unattractive for people. sure theyll have more disposable income since there is no income tax, but it doesnt outweigh the cost to the economy of having people be reluctant to spend their money on things anymore. the effect of this type of system isnt known completely, but i could see it causing an absolute disaster. afterall, what difference does taxing incomes matter when people are getting laid off and have no income to be taxed at all because their businesses arent selling as much because people are too cautious to spend their money due to the national sales tax? thats an extreme case but it could very well happen depending on how people react to the change.
in my opinion i doubt we'll ever see anything other then the system we have now