From HereWhen designing a tax system and evaluating tax proposals, policy analysts have at least four goals in mind:
Efficiency: The tax system should distort incentives as little as possible (and, in the case of externalities and Pigovian taxes, correct incentives when necessary).
Intergenerational equity: The tax system should raise enough revenue so current generations do not unduly burden future generations.
Egalitarianism: The tax system should try to achieve a more equal distribution of after-tax incomes.
Stabilization: The tax system should help maintain the economy at full employment.
The current debate over fiscal stimulus involves trading off these goals. The stimulus package being discussed is mainly aimed at achieving goal 4, but it does so at the cost of sacrificing goals 1 and 2 to some degree. Efficiency is sacrificed because the phase out raises effective marginal tax rates and because the higher future taxes that result from the extra government debt will likely be distortionary. Of course, the phase out is there in order to achieve goal 3: This is the classic tradeoff between efficiency and equality.
Personally I tend to tune out folks who only focus only on the benefits or costs. I'm a lot more willing to listen to someone who talks about the tradeoffs that any tax or policy makes.
People in favor of less progressive taxes tend to emphasize efficiency and stability. They usually reference supply side economics. There's certainly room for a good discussion about how much (or little) damage tax increases do to businesses and economic growth. That's one area of discussion. Another area is to weigh the goals (more equality) against the costs(slight slowdown, maybe).
Hopefully your comparison of flat taxes and progressive taxes is about the US's situation. If so, there are some pretty compelling arguments against flat taxes. Right now the middle quintile pays 14.2% effective taxes. PDF -> Link. The top 10% of tax payers pay 60% of all taxes. PDF again Link. Let's assume that your flat tax will be revenue neutral, let's also discard Laffer silliness as it's not applicable (Google what Mankiw has to say about Laffer curve folks, also his analysis of the Bush Tax cuts shows they didn't meet the Laffer standard). Any revenue neutral flat tax will not be that different from the rate the top tier is paying, since they pay the majority of the taxes.