I think a true flat tax (more likely a flat tax w low income exemption) on income would likely result in the government taking more money from the super rich while keeping the status quo for the rest of us, resulting in a better budget. Uncle Sam could look at the average effective tax rate for a band of income and extrapolate those #s at the flat tax rayes.
However, a true flat tax would remove many jobs for tax preparers, accountant, retirement plan experts (no more deductions for donations, retirement plan contributions, etc.).
With all the jobs that would be lost, and the lobbyists fighing to protect them, it will never happen. The IRC will get more complicated, and likely see a bump in income for top earners. They could also impose a alternative minimum gross earnings tax for those making over a certain $ threshold. Many states already have a gross receipts tax on businesses. Seems to fix most of the tax games. Again, get past the lobbyists and taxpayers...don't think it will ever happen.