Good question. I don't think the mathematics of a flat tax makes sense. Incomes don't really scale linearly, but a flat tax does. By that I mean that 1) the basic expenses of the rich and poor are relatively similar (we all need food, shelter, gas, etc), so the poor person is already burdened heavier since they have to spend a vastly higher proportion of their income on everyday expenses. A flat tax is therefore disproportionately higher the less income one has, not proportionately equal, as proponents claim. 2) Richer people have a lot more to invest (which is good, we need investment), but growth on investments is exponential. So, you have a case where tax scales linearly, while incomes scale exponentially, which leads to the rich getting way richer, and the not-so-rich and poor becoming stagnant. This is basically the situation we've had since the marginal rates have fallen so heavily since the mid-late 80s. As evidence, wealth aggregation has accumulated to levels not seen since just before the Great Depression; the richest 1% now hold something around 23% of our national wealth. This isn't sustainable, since the middle class needs to spend to keep the economy going (growing, too).

Furthermore, there is a stabilizing benefit to a progressive income tax. When times are good, more people enter into the high tax bracket, and speculation is discouraged. When times are bad, the tax is automatically scaled back for people whose incomes have dropped. The effect is a calming one, since a booming economy can be restrained, and a lagging one can be kick started (with perhaps some additional tax cut or spending stimulus).

I am all for getting rid of as many tax credits as possible, as I don't think the gov't should be telling us what to do. I would prefer a lower tax rate to my mortgage deduction. I think the gov't should tax behaviors that are basically considered universal negatives (smoking, pollutants, etc), but that they shouldn't actively encourage this or that specific behavior by paying us for it. For example if they want a fleet of cars with better fuel economy, raise the gas tax, don't give the guy with a Prius a tax credit. That's my two cents on taxes.