And what proportion of taxpayers would be sufficiently disciplined to make sure they have funds on hand to write that check at the end of the year (or quarter,

And what proportion of taxpayers would be sufficiently disciplined to make sure they have funds on hand to write that check at the end of the year (or quarter, or month)? Maybe you'd have the foresight to get a 6-month CD, then pay your tax with the money at the end of that period (and pocket the interest), but the majority of your compatriots would have long since spent it -- whether on healthcare or a holiday in Vegas, either way it's gone.