The rent, ladies and gentlemen, is too damn high.
This is the definite problem with Mr. Cain's plan.
Now, it will likely improve the economy (for a while) by simplifying tax collection, and so forth, and there are lots of nifty benefits. But the thing is, the reason for America's spending problems (spending problems that are replicated in nearly every Western country) is not that America's tax income is too low. It is not even that Democrats or Republicans are profligate spenders. The difficulty is that government does (tries to do) too many things all at once.
This is why, in fact, why the Paul plan is superior to this plan: it strikes at the root, not at the branches. The root of all our problems, politically, is not the height of taxes, it's the size of government.