Flat taxes will straightforwardly result in the rich paying less, not more, and as Jeff says, probably no less complication. I think this is bad.

Elaboration:

Well, tax law is really all about effects. The global depression of the 30s proved that you can’t keep order without paying off most of your citizen-peasants not to starve and revolt. Call it a social safety net, or a tax-and-spend welfare state. Of course, the government should make sure to get the most labor and value out of all of that other people’s money. Communist governments tried to do it all themselves, but for better efficiency, capitalist governments hire interchangeable subcontractors.

The wealthy always insist they can handle paying off the populace perfectly well themselves, but the fact is that you can only spend so much on luxury goods, and the wealthy just aren’t quite generous enough about throwing their money into the bottomless cash sinks of art and science. Currently the US is biased to science rather than art, as our government hires relatively few people as painters, sculptors, musicians, composers, poets, and writers, and more as doctors, physicists, and mathematicians. Our art is based on individual citizens buying luxury goods: TV, video games, movies, rock music, and sports. So spending taxes shapes the world.

But of course, instead of going through all the hassle of taking money and spending it, you can just not take very specific pools of money. In a high-profile disaster, eighties tax policy made real estate investments ridiculously good deals, even if the investor could only find some desolate swamp to buy; eventually that bubble burst in the notorious S&L scandals. In low-profile successes, the maze of deductions has no doubt guided all sorts of US development and commerce. It keeps lots of churches going, and even helped shepherd them through the cultural changes of the late sixties. Ending these deductions will force the economy, particularly investors, into a free market. That will obviously force tax spending to change as well, and I have no idea what will be the good and bad net changes.

I agree that simplification is a very good thing. It will force the social engineering to be done more openly, as well as saving everyone great hassles and eliminating lots of bad loopholes. But this can be done by eliminating either of income and payroll taxes. Without an income tax, workers could simply spend everything they took home from the job, and payroll tax calculation and payment would be centralized. Interest, dividends, and other income would be taxed separately, allowing different rates for them than for real wages.

But on to my main argument with your statement: the tax system is inherently and inevitably social engineering. Progressive taxes tend generally to push society into a bell curve and flat taxes tend to let it spread more, possibly even creating a 2-class caste system. Poor countries like Mexico have always had a few wealthy people, a lot of poor people, very little middle class, and high crime and oppression, and there is evidence of this happening in the US as our middle class shrinks. Therefore, I recommend tax = net_income - (tax_rate*log(net_income)). The equation’s a bit uglier, but it makes a lovely curve.