Because an income tax is subtractive, if you change one of the tax rates being applied, the other taxes still collect the same tax because the amount being taxed (gross income) doesn't change. So if a person has $50,000 income, adjusting the income tax rate or the payroll tax rate does not change the fact that those taxes are being applied to $50,000. The inclusive rate is natural and correct for a subtractive tax like the income tax.
A sales tax is additive, if you change one of the tax rates being applied, the other taxes collect more or less tax because the amount being tax (gross payment) has changed. The inclusive rate is unnatural and incorrect for a additive tax like the sales tax.
Example:
Tax Inclusive General Revenue Rate 14.91%Combined OASDI & Medicare Rates 8.09%Total Rate 23.00%
Gross Payment on $1000 item $1,298.70Amount going to General Revenue
(14.91% x $1,298.70) $193.65Effective Tax Exclusive General Revenue Rate 19.37%
Now with an increase in the combined OASDI & Medicare rates (the ones set by the SSA) and keeping the statutory 14.91% general revenue rate.
Tax Inclusive General Revenue Rate 14.91%Combined OASDI & Medicare Rates 10.09%Total Rate 25.00%
Gross Payment on $1000 item $1,333.33Amount going to General Revenue
(14.91% x $1,333.33) $198.80Effective Tax Exclusive General Revenue Rate 19.88%
So you have a general revenue tax increase without changing the rate. I really don't see how this situation is tenable.
This clearly illustrates that exclusive rate is the only correct rate to use for a sales tax. If you want to compare the tax exclusive sales tax rate to the tax inclusive income tax rate, the conversion is very simple.
Now a lot of the FairTax supporters have become very dogmatic about the inclusive rate and y'all will be inclined to go into attack mode. Instead I would suggest you take a close look at what's going on in this illustration and ask if having a rate that has a slight relation to the current system that y'all want to abandon and have forgotten worth the effect shown here. I haven't seen this effect shown anywhere previously (it may have been, but I haven't seen it) so I'm guessing most of you weren't aware of it.
Changing to the exclusive rate doesn't change any of the fundamentals that y'all believe make the FairTax such a great idea, but it's obvious that the way the rate is expressed will have to be changed before this becomes law. The sooner the better for your cause.
They may want to, or claim to want to abandon it, but suspiciously, you won't find bigger defenders of the income tax either.