Beyond a boost for charity, though, the stunt showed anyone looking skyward at 2:32 p.m. on Market Street that, whatever you think of his politics, Philly's leader possesses some high-wire audacity.
"You got heart, Mike!" one of the crowd of about 200 who gathered to witness the event yelled after it was over eight minutes later.
Calm as a matador, Nutter smiled a smile of Zen self-possession.
The day started about 7 a.m. with the first of about 70 daring hearts going over the side of the building 21 stories above the street.
To handle the technical details of harnessing the participants, Outward Bound hired Over the Edge, a Canadian company that helps charities raise money by staging urban rappelling experiences.
The thinking is, anyone can host a charity chicken dinner. But for true sizzle, Edge executives suggest, have people make like Batman and skitter down a high-rise.
Each rappeller was required to raise or donate $2,000. By the end of the day, the event had brought in $157,000, with more expected, according to a spokeswoman for Outward Bound Philadelphia. Local corporate donors contributed, as well.
"This creates visibility for the program," said Katie Newsom Pastuszek, executive director of Outward Bound Philadelphia. "The School District of Philadelphia can't afford to pay the full ride for kids to participate in Outward Bound, so we need to raise more."
Before Nutter showed up on the roof, Brian Hughes, site safety supervisor for Over the Edge, assured anyone who would listen that the mayor would survive the descent.
"I can guarantee the mayor will still be around tomorrow," he said, explaining that Nutter's full-body harness would be held by two extraordinarily strong rope lines, each capable of holding 6,000 pounds.
Like other rappellers, Nutter was required to don cloth booties so as not to scuff the glass building.
"Just get to the edge of the building, and then we'll bring in the stand-in," Nutter's security chief, Highway Patrol Sgt. Michael Walton, shouted jokingly to the mayor.
Nutter, who had heard a number of people try to dissuade him during the week, was having none of it. "So, what are we doing?" he said as he stood on the lip of the roof in a dark-blue Outward Bound fleece and a helmet equipped with a camera.
William Penn, standing atop the nearby tower of City Hall, had his back turned to the whole thing, as though unable to watch.
Seemingly unawed by the circumstances, Nutter - who had done rock-climbing, but who had never rappelled before - said simply, "You have to trust the people you're working with and trust the gear. I'm very, very confident."
Then he was gone.
On his way down, Nutter stood on the windows and - as though campaigning in a novel way - waved to stunned workers in the building in which he once worked at an investment firm.
When he landed, Nutter acknowledged applause from the crowd. Asked whether he'd rappel from the Comcast building should anyone suggest it, he shook his head, saying, "I will help other people do that."
Contact Alfred Lubrano
at 215-854-4969 or firstname.lastname@example.org.