A flat tax per se is not necessarily a bad idea - it just depends how it's implemented. That's really something for another thread, but simply calling a flat tax fuckwittery is IMHO foolish.
It's a fucking terrible idea. It completely fails to recognise many things:
1. The utility of money reduces with income. The first few quid that get you housed, fed, warm, clean, clothed and mobile are the key to survival. The next are used to improve the quality of your housing, food, heating, clothing, mode of transport and to chuck a few luxuries in like holidays or better entertainment. Spending after that becomes increasingly trivial, such that people are willing to spends tens of thousands to fly their hairdresser half way around the world for one night.
2. Taxation pays for the economy that taxpayers earn money from. If there were no reason to be based here, the rich would go to Somalia where there are no taxes. Those that reap disproportionate reward from the economy should pay disproportionately towards its costs (of providing a literate, numerate, and often highly trained, healthy workforce, a criminal justice system overwhelmingly focused on protecting property and enforcing contracts, a sophisticated communications and transport infrastructure, a government that pimps us out around the world and underwrites their bad gambling debts, you know, those little things).
3. Income inequality reduces the capacity for capitalist growth. Rich people save their money by investing in future bubbles which will burst and ruin us all, poor people spend it on things that other people need to be in work to provide, like nice food and clothes and going out more often. It is harmful in every conceivable way to allow very large income gaps to arise, and taxation is the way the gap can be prevented from getting too wide.
4. If what you actually mean is if we had equal wages for all, then ignore the above. I agree.