WHY HAVE a lame-duck session of Congress at all? It's a good question. The original Reagan administration position was that Congress should meet to pass appropriations bills, and some Republicans added that it should address the Social Security issue. But the lame-duck session will almost surely do neither. The president likes to chastise Congress for not passing appropriations bills, but the fact is that Congress has already produced the equivalent of a whole set of appropriations bills in the budget resolutions. To a much greater extent than in the days before the current budget process, Congress knows how much government will spend and take in, and has made intelligent decisions on the basis of that knowledge.

True, government by concurrent resolution is not the ideal process; and Congress may pass some appropriations bills. But the one most likely to pass -- defense appropriations -- may contain some provisions not much to the administration's liking. Opponents of the Dense Pack deployment of the MX missile, for example, will try to kill it by amendment.

As for Social Security, Speaker O'Neill put the kibosh on any lame-duck action with just a few words on election night. That's a positive result in at least one respect: it means that the Democrats, who clearly will control the next House, will have to take joint responsibility for any congressional action on Social Security. The potential, at least, is there for taking Social Security out of partisan politicking and into the realm of constructive policy-making. But it won't happen until sometime next year.

In the meantime, Congress has much pending legislation before it. The most important -- and perhaps most difficult -- is the immigration bill. Congress may pass the gas tax increase and highway construction measure the speaker and Senate Majority Leader Baker jointly endorsed. It is less likely to pass the Clean Air Act revision or regulatory reform sought by the administration; these are complex matters, far from passage, and Democrats have every incentive to block them until the more Democratic House is seated in January.

The administration will probably lambaste the Congress for inaction on appropriations and other bills in the lame-duck session. But lame-duck sessions are seldom productive, and if Congress manages to do well by one or two major pieces of legislation responsibly this December, it will have had a lame-duck session above the historic average in productivity.