1. The United States is on an unsustainable fiscal path.I'm not an economist - micro nor macro - but having lived in countries with a VAT (and subsidized health care, staple foods, etc. to offset the impact of a VAT on lower-income people), I can say that it didn't seem too horrible. Of course, I wasn't earning any real income, nor did I own property of any value, so my recollections are not the best yardstick. However, the idea of shifting revenue streams from income taxes to a VAT is somewhat appealing (doing it in addition to income taxes is not very appealing at all), and would have the additional benefit of making it real easy to determine your cost at the cashier (since all the taxes would be added into the price already; thus the name "Value-Added Tax").
2. For whatever reason, long-term interest rates don't reflect this problem. There will either be a sudden collapse of demand for government securities, or the current market already is figuring we will get a VAT. Either way it is more revenue for the government or a Greece-like scenario writ large.
3. I would prefer spending cuts, but voters seem too irrational to be willing to cut spending; here the libertarian argument comes back to bite us on the bum. They might be willing to cut spending once a financial crisis arrives (though maybe not), but then there will be days or only hours for decisive action.
4. We could, for now, wait and postpone fiscal reform. That means encountering a sudden collapse some number of years from now. We will then clean up the budget in some way, but under a TARP sort of mood rather than what we might do today.
5. We'll get a better deal, and make wiser decisions, if we do it today rather than in a panic. Plus another financial crisis would prove deadly to both the budget and to the quality of economic thinking.
6. There exists a credible bipartisan deal which involves at least half the VAT revenue for deficit reduction, combined with cuts, or slower increases, in marginal tax rates on income and perhaps an elimination of the corporate income tax. Spend some of the rest on health care for the poor, if that is the deal on the Democratic side.
There are, of course, the regular arguments that one may hear about VAT: that it is a greater financial burden on the poor, since a larger percentage of their income would be going to pay for VAT. However, that can be ameliorated by various measures, including subsidies on staple products or VAT-refunds (or something even more clever). If the country shifted to VAT, it would definitely simplify the tax code, creating fewer loopholes, lowering the income tax rate (possibly all the way to 0% for earned-income). This would diminish the size of the GOP-despised Internal Revenue Service, since there would be fewer needs to conduct audits on tax returns (it might even get rolled into a single agency alongside the Security Exchange Commission, so that one agency is in charge of all financial-related policing).
[Disclaimer: I'm writing this while hyped up on coffee and tea, so my mind isn't the clearest at the moment. Still, if you would like to write in to comment, feel free to do so.]