With usual and customary business deductions, wouldn't that bring the effective business tax to zero?
No. Lowering the tax rate to 15% would not reduce the effective rate to zero. Take at look at the following example of how effective tax rate is calculated for corporations. In the following example it would reduce the effective rate from 30.9% to 16.9% for a taxable income of 5 million. This assumes that all loopholes remain the same which they wouldn't. The 15% is a bit low but I basically agree corporate rates need to come down and some loopholes need to be closed. If done properly, it will increase treasury revenue and bring some businesses and jobs back into the country.
Corporate Tax Table Tax Rate:
First $100,000 10%
$100,001-$500,000 15%
$500,001-$1,000,000 25%
Over $1,000,000 35%
Annual Pre-Tax Earnings = $5,000,000
Total Taxes Paid = ($100,000 *10% + $400,000 * 15% + $1,000,000 * 25% + 35% * 3,500,000) = $1,545,000
Effective Tax Rate = $1,545,000 / $5,000,000 = 30.9%