THE LONG QUARREL between the rich nations and the poor - the North and South - doesn't seem to have made such progress in Manilla. The representatives of 159 governments met there for a month and, when it ended last week, they had very little to show for their labors. It was UNCTAD, the fifth United Nations Conference on Trade and Development, a forum that-like the U.N. General Assembly-has become an invitation to the least wealthy and powerful to vent their grievances.The character of this latest session raises a question whether UNCTAD itself is not a bad idea-whether, as an institution, it does not generate an atmosphere of ideological rigidity and confrontation.

But the answer is that, on balance, it's bette r to have UNCTAD than not. The grievances are real, and the world does not escape them by refusing to listen. While this latest meeting accomplished little, there were signs here and there of a new series of realism.

The rich countries are worried about the coming recession and the instability of their currencies. They are not eager to talk about large new aid plans. At a time of high domestic umemployment, they are not anxious to increase imports from the developing countries. But they all know that, without strong markets for their own exports in these same development countries, the recession of 1974-75 would have been much more severe. Precisely because another recession is now on the horizon, the North has good reason to think carefully about the prosperity of their customers to the south. Where altruism fails self-interest is often a very adequate substitute.

On the other side of the equation, the poor countries are making a slow and painful recovery from what you might call an overdose of OPEC euphoria. When OPEC suddenly changed the world's econimic balance of power five year ago, a great many countries with low incomes saw it as a universal model. There was talk of little OPECs in everything from bauxite to bananas. But, of course, they haven't worked. In some governments that has only raised the sense of frustration and dependence. In others, more profitably, it has revived interest in conventional trade strategy.

The developing countries need, above all, two things. They need better access to the markets of the industrial countries, in a time of rising protectionism. They also need more development capital, for which the World Bank and the other international institutions are now the main conduits. For Americans, the right response to UNCTAD is not to try to win the endless debates over theories of imperialsim and accusations of exploitation. The right response is, instead, to keep expanding the international banks with American dollars that are matched by other wealthy nations. The right response is especially to keep this country open to foreign imports - including textiles, shoes and transistor radios. Open markets, incidentally, benefit American consumers at least as much as foreign producers.