Consider what would happen if the IRS was efficient enough (through these automated tax accounting systems) to not get an interest free loan from working America for an entire year (ie dynamically modifying every pay period how much was collected in federal tax withholding based on estimated total year tax liability); the federal government would have to be more judicious about its cash flow.
You'd think that's something Republicans and small government advocates could get behind. Go figure. "Fiscal conservatives" in name only.
EDIT: For the pedantic: s/IRS/US Treasury/ in my comment.