If you want to understand why one would or should support Herman Cain's 9-9-9 tax plan--not that I endorse it--I strongly recommend Art Laffer's positive review in the Wall Street Journal. With "Cain's Stimulating '9-9-9' Tax Reform," Laffer (famous for the Laffer curve and popularizing supply-side economics) first laments the "ever-more arcane" tax code that we have.
"...the sole purpose of the tax code was to raise the necessary funds to run government. But in today's world the tax mandate has many more facets... income redistribution, encouraging favored industries, and discouraging unfavorable behavior...
Three federal taxes at 9% that would raise roughly $2.3 trillion [the current level] and replace the current income tax, corporate tax, payroll tax (employer and employee), capital gains tax and estate tax."
Along with simplifying compliance, Laffer emphasizes the increased incentives for productive activity. In other words, if you face a 9 percent marginal tax rate, rather than a 35 percent one, you'll be more likely to work.