Actually, you're not. There's a state sales tax in 48 states, and that hasn't caused a black market. Then there's the fact that the seller doesn't benefit from the tax break and runs the greatest risk of being caught. Sure, there'll be some dodges, but compare that to how many people dodge their taxes now that you can do it by yourself.
Saving money for later instead of spending it is far wiser and is good for the economy, as it lowers interest rates on loans (banks have more money to dole out) and it encourages investment, which isn't taxed and leads to booms. Not to mention that when people start saving money rather than spending it, they start looking to the future for things like home ownership, another thing that benefits the economy.
Now, I am currently aware that discouragement is not the sole reason for taxation. It is, however, an undeniable side affect of any tax. Look at the luxury tax. When it was enacted, it didn't collect more money, it just meant fewer people bought luxuries. Now, as we have seen, whether you save, invest, or spend, it benefits the economy. However, if you're discouraged from making money in the first place through income tax, it hurts the economy, so why don't we tax something other than income? Taxing spending at the retail level won't do that much to discourage spending, as spending is already taxed through embedded income tax. However, eliminating the income tax will encourage increased earnings, leading to an economic boom wherever it goes.
We agree on something? Pinch me. Personally, I think this alone is enough to justify eliminating the income tax, but then there's all those other reasons that make it all the sweeter.