The argument in favor of the Concorde, the clincher that convinced so many people that this super-speedy airplane was worth whatever environmental risks in involved, was the Crucial Business Deal.

Your employer might not consider your time worth $500 an hour, but somewhere there were business people who negotiating skills were so blindingly brilliant that the need for them to appear acorss the ocean in 3 1/2 hours, instead of six or seven, or instead of an instant telephone call or cable, was worth that.

If you have the kind of a job where your employer wants to know why you didn't walk those dozen blocks instead of taking a $1.10 taxi ride, it's exciting to think that somewhere things are different.

This work-ethic fantasy was shattered, though, by the goings-on at the French embassy on New Year's Eve. There was high merriment, champagne, applause and kisses from the French ambassador's wife for a dozen people who had just succeeded in using the Concorde as we all knew deep down that it was born to be used - as a great big expensive mechanical toy.

The guests of honor at the party thrown by Ambassador and Mrs. Jacques Kosciusko-Morizet were the survivors of "Encore III - An Extraordinary Voyave Through Time," which was a three-day round trip to Paris on the Concorde, timed so that one could spend midnight first in Paris, then on the airplane, and finally in Washington, at the French embassy. (The eight other people who started out on this voyage but did not return did not land in heaven: They stayed in Paris, which is what Oscar Wilde wished instead of heaven for good Americans.)

The travelers had paid $3,235 each for the trip, hotel rooms, limousine service and meals for their day-and-a-half in Paris and Friday night here. At that, the price had been marked down from $4,850 - not, said organizer L. Bradley Stark, because the tickets weren't moving, but because designer Pierre Cardin had canceled meeting the group, which would have been, presumbaly, a $1,615 experience.

The regular Air France round-trip fare to Paris is $687 for coach and $1,379 for the first class on conventional aircraft, and $1,654 on the Concorde. Air France charged the full Concorde fare for each of the New Year's passengers.

"I'm sick of hearing about how expensive this is," said Stark. "It look an enormous amount of time and travel to set this up. We're offering an adventure. At the price, people are getting a bargain."

The passengers, as they concluded their adventure at the French embassy, seemed to agree. "I've put away a case of French champagne since Christmas," said one woman, who added that she was feeling somewhat weak. "It's been perfect."

The passengers said they had gone to sleep when they first got to Paris, and then most of them did some sightseeing on Thursday, although some stayed in their hotel because it was raining.

Still, they were all so delighted with the arrangements that there was a general demand to see the French embassy's chef, in order to applaud him for having prepared the cheese trays and strawberries with whipped cream that made the desert of the meal they had begun on Friday night in Paris. The first course had been at the Inter-Continental Hotel, and the second on the Concorde.

"I'll never go any other way," declared Fred M. Allison of Midland, Tex., whose card has the words "Retired Millionaire" discreetly engraved under his name. "I'm still trying to dry off from the shower I had in Paris."

Not everybody was in that financial bracket. "This is my new car," said Margaret Macklin of Delray Beach, Fla., a third-grade teacher who went with her husband, who is retired from the Coast Guard, and their son, who runs a movie theater in Boca Raton.

"We had saved up, and we even had the car all picked out, when we decided to do this instead." But this impulse seems less impetuous when the Macklins tell you that they have had two previous international vacations this year - one a trip around the world in 46 hours and 50 seconds, on Pan American, and the other in which they took the British Concorde to London, stayed three days, and then took the French Concorde home.

"That's the way to travel - get it all over with quick and get home," said Macklin. "For gosh sakes, I can't afford to be away. I've got grass growing and grapefruit to pick. Besides, this is the first time I've had to have shoes on since the last time I was in Paris. I had to go and buy a pair of socks so I could make this trip."

Asked what he does at home, now that he is retired, Macklin announced, "I'm a gourmet cook." When he was asked upon departure last Wednesday at Dulles whether he wouldn't be eating some good meals on Air France and in Paris, he looked skeptical and revealed his secret supplies, "just in case." They consisted of two sandwiches, "ham and cheese on Hollywood bread." Mrs. Macklin carried seven tangerines in her purse, but gave two away before departure.

Although Macklin had said on the flight from Florida to Washington, that he was ready to go home, he said on his return to Paris that "This is the way flying is supposed to be." He and his wife and son are all licensed pilots, and his job in the Coast Guard was to "fly the Secretary of the Treasury around during the Truman administration. This reminds me of the old days - a limousine to meet you and a hotel all ready, instead of having to get a cab, if you can find one, and making your own reservation in a motel."

Macklin had requested a rum and Coke on the trip over and been told that there was no rum aboard. He replied that he would never fly the Concorde again if there weren't rum on the return trip and there was, supplied by Encore III.

The only other special requests from passengers were from one woman who wanted a map of Paris, and another who wanted Concorde postal cards. Both were granted.

Stark, who was formerly in Detroit in "the incentives business - like thinking up incentives for the salesman who sells the most Cadillacs" - formed the Encore Marketing Co. in New York for this trip.

He said he hoped to repeat this trip, and has other "special projects" in mind "which had better be kept secret right now." He and his associate, David Wood, accompanied the travelers, and also made eight trips to Paris themselves to set it up.

The original plan was for the plane to slow down to the speed of the earth's rotation at midnight "in order to make time stand still," but Dulles regulations requiring planes to land by 10 p.m. did not stand still, so the plan was abandoned.

At midnight on the plane, according to Mrs. Macklin, "We went up and down the aisles, learning to kiss in the Frency way."

According to Macklin, this is nothing special. "Just a peck on both cheeks - nothing," he said.

"This could have gone either way," he said, pleased with the passengers' evident satisfaction with their "adventure."

"How could you surpass this unless you went to the moon in person?" asked Mrs. Robert B. Clarke. She and her husband, a Grosse Point, Mich., psychiatrist, took the maident voyage around the world of the Queen Elizabeth 2, and are thinking of taking the New Year's Eve Concorde trip "again in 1981 - it will take me that long to incuperate. I'm beat."

For Gisela Stern, a widow who lives in St. Louis with her 10-year-old son, the clinching argument for taking the trip came from her sister, who said "You only live once." "And I thought," said Mrs. Stern, "that's right."

Mrs. Stern's parents were visiting her from Munich, Germany for Christmas, and she left her father home with her son while she and her mother, Elizabeth Hofbeck, flew to Paris and back.

But meanwhile, what of those Crucial Deals? What about the French government itself, which has diplomats commuting between Washington and Paris all the time?

The rule is that only the highest-ranking French diplomats may travel by Concorde, which means about 10 of the 40 or so diplomats posted in Washington. The others travel by regular planes, but of course thye must travel Air France.

Since Air France no longer flies its regular planes from Washington, this means traveling to New York to get a flight home, a connection which involves either taking a much earlier flight from Dulles to Kennedy airport, or changing airports in New York.

One diplomat mentioned a 19-hour trip home from Washington, a journey that took eight or 10 hours, including everything, before the arrival of the Concorde.

But the French ambassador, who has made six round trips on the Concorde, declared, "It has changed my life. I arrive 10:30 Paris time, and I can go to sleep. I never slept on the plane. If I come Saturday night, I am rested, and then I can spend Sunday with my family."

Mrs. Kosciusko-Morizet said that she, too, love the Concorde, but it makes her less productive. "I used to do lot of needlepoint between here and Paris; now the most I can do is write a few postcards to friends."