The reason the fair tax has to be so high(23%, 30%, or some even say higher) is because its all inclusive, it replaces all taxes(income, payroll, corporate, estate....ect.) Some of the major benefits include the dropping of the corporate tax, the US currently has I think the second highest corporate tax in the world. Getting rid of it would do a lot to stop all of the large corporations from leaving the US and probably get quite a few to return. Another benefit is a vast simplification of the tax code, saving everyone money all around. And a third benefit being actually getting money from the black markets, illegal immigrants, and drug dealers for the fact they have to spend that money they are getting on "goods" at some point, all the current money they make goes unseen so therefore untaxed.
Now the Cons and there are some big ones. First the current plan puts a tax on all new goods that includes cars and homes, can you imagine paying 23% on top of the cost of a new car or home. Although people will eventually adjust to the price increase the first 5-10 years would be absolutely devastating to the construction and auto industries(probably all industies though especially retail). Although I think there are some transition programs, there surely would be a inevitable recession type backlash because of it. Another problem I have is it makes it easier from the government to change what to rakes in, the congress could slowly jack up the tax rate at any time if they run short on funds. And lastly my main beef with it is the fact that the prebate program would effectively put everyone on a welfare program.
Now as much as I'd like to see the income tax repealed it's a pipe dream, it would take probably 10-15 years of a paul-like presidency and congress that agreed. What I do think is possible though is a mixture of the current system and the flat tax. Drop the Capital gains, Payroll, and income tax Period! You keep the Corporate tax but knock 10-15% off to lower it between 5%(small businesses) and 20%(large businesses). Keep the Death tax but change it to 25%-35% of all estates worth over 5-10 million. Raise the age of Social Security programs by almost 10 years, 67 to 75(The original 1933 program almost makes sense, you didn't get the money until 65 but the average life span was around 58) And finally instate a national sales tax of around 6%-8% and put a lock on the percentage that it can't be raised. Do that along with some government spending cuts and you might have an actually workable tax plan.