I have to admit it - I thought I was generally in favor of tax reform that would simplify the tax code, lower rates, reduce deductions and loopholes and still increase total tax revenue. Then I looked at some IRS statistics. (Off topic - one difference between Republicans & Democrats is that we generally think data matters more than opinions.)
This chart shows the difference between Adjusted Gross Income (AGI) and taxable Income (TI) as a percent of AGI. The horizontal scale is AGI on a log scale to accomodate the full range of incomes.
What I see in this chart is that deductions and exemptions generally benefit low and middle income taxpayers the most. At incomes of $10,000 roughly 75% of AGI is eliminated, leaving a very small (or 0) TI and tax liability. At incomes of $100,000 deductions are about 30% of AGI. Although the deductions are larger in absolute amounts, they cover a smaller portion of AGI and thus provide a relatively smaller tax savings. At incomes over $1 million, deductions are less than 15% of AGI and have an even smaller impact on TI and net taxes.
Especially at lower incomes, these deductions and exemptions are a big contributor to the progressive nature of net taxes (for incomes up to about $1M). Nominal rates aren't necessarily well structured, but net taxes relative to AGI are lowest at lower incomes because of the relatively large deductions and exemptions.
So those who want to lower rates across the board and pay for the lower rates with fewer deductions would simply move the tax burden from high income levels to middle and lower income levels. At low incomes, a lower rate will not make up for the loss of relatively big deductions. At higher incomes, the savings from lower rates will far outweigh the cost of giving up a relatively small deduction. Any attempt to mitigate these dynamics by only limiting deductions for high income levels will fail to raise any significant tax revenue - that's not where most of the deductions are used.
Two more points: This is all based on the difference between AGI and TI. It ignores all of the adjustments that go into AGI - for example, business gain and losses that are reported on personal tax returns. Deductability of business expenses is another topic to investigate and probably contains loopholes & deductions that may matter more at higer income levels. But most of the proposals I've heard about tax reform are talking about limiting deductions for mortgage interest, property tax, non profit contributions etc. Second, in 2009 deductions were slightly higher across the board than they were in 2000 before the Bush tax cuts - another reason that simply letting the Bush cuts expire will hurt everyone, not just the wealthy.
We need tax reform, and we do need to "expand the tax base". But we need to expand the tax base upwards at high incomes - not expand the tax base by taking away deductions that mainly benefit people making under $100,000.