First off, a point I will repeat once again is that progressive taxation doesn't (really) increase the tax onhigh income earners because they have the power and ability to pass those added costs onto other people through higher cost goods and services or through lower payments and benefits to their employees. What this means is that only the people who have limited ability to pass the costs along (generally the poor and middle class) see the negative effects of higher taxation.
Now, even if you suppose that you could tax high income earners through progressive taxation why would you want to?
People who are successful at building their own wealth have demonstrated a high level of skill at efficiently using money to build products or services that are in high demand; and their money tends to be fully re-invested in the legitimate economy through the purchase of goods, services and investments. By taking money away from these people you're taking money away from the people who have the best understanding of how to re-invest the money to maximize the growth of the economy, and you're preventing the money from being passed along the "Food Chain" to companies who employ people and buy goods and services from other companies, who employ people and buy goods and services from other companies, and so on ...
In my opinion the true benefit of a flat tax system is simplicity ... Everyone just adds up all of their income and all of their taxable benefits multiplies it by the tax rate and mails a post card into the Revenue Agency (depending on your country) where they verify your numbers against the values your company sent in and issue a bill or credit appropriately.
When you eliminate loopholes and stop giving tax credits or deductions to certain individuals to promote their way of life the ammount of bureaucracy associated with collecting tax is greatly reduced; and a simple and straightforward tax and legal system is a huge advantage for companies who want to do business in your country.
