When will the PLO walk an extra mile?

Ever since 1947 Arabs have been rejecting others' initiatives on behalf of Palestinians. Now the Palestine Liberation Organization has done it again in Algiers, where President Reagan's Mideast peace plan got short shrift from the first Palestine National Council session since the PLO was forced from Beirut. Are Arab leaders ever going to get off their flying carpets, look realism in the face, and dare a constructive response to somebody who's trying to help?

To be sure, Arabs can argue they might have been taken advantage of if they had worked with what was offered at various stages in the past.

They can cite the record of Zionists before the modern state of Israel was established as well as Israel's expansionist settlements in occupied territorities today. They can cite doubts about doing business with a Prime Minister Begin, who is no slouch at rejecting Reagan offers either. There is not much confidence in the Mideast air when a President of the United States can offer Israel a security ''guarantee'' and a prime minister of Israel can reply, ''There is no guarantee that can guarantee a guarantee.''

But would the Palestinians really be worse off now if Arabs had cooperated in past efforts toward solution? And will they be better off in the future - or will they see more and more territory gobbled up - if they continue to refuse to cooperate now?

The ''what ifs'' at least have to be asked. For example:

What if the Arabs had accepted the United Nations partition of Palestine in 1947? At least some Palestinian lands were set aside then.

What if Arabs had accepted the efforts of Eisenhower emissary Eric Johnston to bring about Arab-Israeli utilization of the Jordan River to help provide homes and livelihood for Palestinian refugees?

What if some Arabs, including the Palestinian Jordanians, had not denounced the Baghdad Pact as a diversion from the Palestine problem but joined the other Arabs willing to cooperate for stability in the region?

What if the PLO had long ago recognized Israel's right to exist in the spirit of the UN resolutions requiring Israel to return to 1967 borders? Could Israel have continued a policy of territorial expansion?

What if Palestinians had accepted a version of the independent Jordanian-Palestinian state that Israel officially favored back in 1974 - and that Mr. Reagan speaks of now?

What if Palestinians had gone even part way through the door opened to them by Camp David?

Palestinians might give a cynical answer to all of the above.

Yet can they really afford to be entirely cynical about the backing Mr. Reagan offers now? Can they be sure they will ever get more satisfaction of their legitimate grievances under some future dispensation?

PLO leader Arafat claims success for his ''moderate'' position against PLO radicals in that further exploration of the Reagan plan is not totally shut off. He evidently is permitted to look further into negotiations with King Hussein of Jordan.

But the only plan receiving a stamp of approval in Algiers was the so-called Fez proposal including an independent Palestinian state - a possibility as remote as a flying carpet at the moment.

President Reagan has gone an extra mile of rhetoric with the Palestinians, referring to their need for something in the nature of a ''homeland.'' Thus he used a favored Arab word without changing his goal of self-governing Palestinians associated with Jordan.

Is such a goal really so bad compared to what the Palestinians have - or don't have - now? Isn't it simply good, self-serving sense for them to go some sort of extra mile, too?