>> deleted: Deliberate design for MBP failure. << They arc with the AL clamshell back, pitting it after years, and the springs and pins fail from normal stresses of handling the unit. They measure about 2 ohms to ground and judging from the pits on the inside of the AL case after years, conduct some significant, likely static currents from the case directly onto the main logic PCB.
The arcing and pitting is very real at clearly-defined round cutouts in the insulation inside the clamshell back cover (see photo links below). There's a remote chance it could be the result of galvanic corrosion from dissimilar Cu/Al metal contact but more likely the pits are from high-voltage ESD across an imperfect contact and what appears, I realize now, to be the intent of these pins in the first place.
Pictures of the pits and the round holes in inside back cover plastic:
Reconsidered, it makes sense ESD discharges find their way to a ground plane on the Main Logic PCB, rather than to any individual components or to any non-ground circuit path.
Continuity of the copper push pins to the case doesn't appear essential, they appear like little 'lightning rods' for the Main Logic PCB, to the extent spots on the interior of the clamshell get eroded away (presumably) by electrical arcing.
Also one of these pins got loose -somewhere- not a good thing, hopefully somewhere else by now, and another pin's shroud has been 'bent' to the extent the spring-housing is cracked, likely by the normal process of handling the MBP with one hand, which stresses the aluminum clamshell case-back and deforms it against a few of these pins.