Yeah, this is what pisses me off about people who suggest radical simplifications of the tax code - reminds me of Steve Yegge's
Shit's Easy Syndrome.
Taxation is difficult. There are a lot of weird things you can do with your money, and a whole industry out of making more of them - dealing with all that requires complexity.
Even more so, as you mention, tax policy is a manifestation of social and economic policy - it's the way you implement those two, and if you're going to do it right, it's going to be complex.
For a small small view of this, consider that as of now, you get a tax deduction for charitable donations. If you simply paid X% of your income, as many suggest, this would remove that deduction - and what do you think that would do to charitable giving in this county, and to the people that depend on the programs of those charities to survive?