One of the men who kept the glass of the twin towers clean was Roko Camaj, whose hazardous duty was the subject of a children’s book “Window Washer: At Work Above the Clouds.” The book, published in 1995, quoted Mr. Camaj saying that “Ten years from now, all window washing will probably be done by a machine.”

On that point, Mr. Camaj, who was killed when the towers were destroyed on Sept. 11, 2001, was not a prophet. Mr. Caulkins said “there may be several dozen” skyscrapers in the entire country that employ robots to clean their windows.

Another reason for the sparse use of robots is that buildings require a lot more maintenance than just window cleaning, Mr. Caulkins said. Equipment is needed to lower people to repair facades and broken windows, like the one that rescue workers had to cut through to rescue the window washers on Wednesday, he said.

“At some point you need workers up on the side of that building,” Mr. Caulkins said. If a building requires a mechanism on the roof to lower a platform for that sort of work, the owner is unlikely to invest in a second, mechanized system for cleaning the glass, he said.

In all, there are about 700 scaffolds for window washing on buildings in New York City, said Gerard McEneaney, a field representative for about 500 washers who are members of S.E.I.U. Local 32BJ, a union representing building service employees. Mr. McEneaney said he understood why the owners of 1 World Trade Center would employ humans to clean the tapered glass facade.

“They want that building sparkling, sparkling clean,” he said.

His members are willing to do the work because it pays well: as much $26.89 an hour plus benefits. Many of the window cleaners are immigrants from South America. “They’re fearless guys, fearless workers,” Mr. McEneaney said.

Having spent nine years on scaffolds in New York City, he said he knew firsthand how scary an ordeal like the one on Wednesday could be. The first time he descended on a scaffold at 1 Penn Plaza, he said, his end of the platform did not stop when it should have, tilting the scaffold.

His partner reacted quickly and lowered his end of the platform to keep it level and avert the need for a rescue, he said. “In a job where you know your life hangs in the balance, you just kind of accept it and rely on your partner and pray that your training has prepared you.”