I pose this question on PEN-L about once a year to see if anyone comes up with anything new. Don't usually get much reaction.
About 8 yrs ago I wrote a piece on this and my own reasons still seem valid:
surpluses are deflationary; deflation is desirable because it slows down economic growth. Slower growth means looser labor markets (less upward wage pressure). Deflation also is good for bond-holders. The bond market is much bigger than the stock market.
surpluses deny revenue to redistributive public spending. this does not follow for defense because the latter is held to be Above Economic/Fiscal Consideration.
surpluses play to the hoked-up hysteria about the long-run fiscal picture, which hysteria helps to bolster demands to shrink or privatize Social Security. Shrink or privatize is the way TMPFITL would like the debate to proceed.
surpluses channel tax revenue into the private capital markets. Those whose Federal debt is redeemed, or for whom buying Gov bonds is precluded, are obliged to buy other financial assets.
surpluses were held to reduce our trade deficit, though this hasn't happened. A lower trade deficit means less foreign claims on U.S. capital/more U.S. claims on foreign capital.
mbs