I tend to favor the flat tax as well, one reason being people would feel it has more of an ethical or equitable (?) basis, and there would be a higher participation rate as a result. The feeling would be if their neighbors are doing it, so do I, and if I don't, their flat rate will have to go up. So there is a direct connection between our actions and the impact on others - and if we do better together as a society, the rate might actually go down.
I also think people at the lower end of the income scale actually want to contribute too, and a flat tax would more likely accomplish that, to feel they are a part of the country. It would also help the country feel as though they are part of one common enterprise where our destinies are interlinked. I can imagine a President saying, if we can produce this amount in revenues this year, we can drop the flat tax another 2%. It would give focus and a reason to celebrate how we conduct our financial affairs. Then if we could just get some discipline over the top line growth on spending, maybe there would be light at the end of the tunnel. Hope over experience, maybe. How retirement accounts, mortgage allowance deductions, charitable giving, health expenses, etc. play into all this gets complicated, I'm sure. But without participation, nothing works and there seems to be ever increasing erosion on that front.