"Used to" is the operative phrase I think, in this day of heavier then ever regulation, certification requirements, and penalties if ever caught. THATS what got me when I saw it, it was like none of that mattered at all to the operator, or he was just clueless.
Today I had a situation, way out in the boonies with no chance of anyone seeing anything, that called out for having a willing carpenter ride the ball up to un rig a sling. The alternative was to lift some planking up into the attic truss assembly so they could use a step ladder, or leave the sling there for a day and get it back later. They eventually rigged a ladder, the other job had a staircase, the flying was totally for the hell of it, and to falacitate de rigging it probably because near as I can tell the spreader bar could only be lowered to the top chord, which was too high to reach, which meant the carpenters had to climb up onto the top chord to un hook it.
This is one reason why I quit using a multiple truss spreader bar about 20 years ago ( I still will sometimes set two at a time, using two chain slings, one rigged about 20" lower then the other, setting the lower one in the usual fashion, nail the stabilizer on it, while the next one is still sitting above the plate line and out of the way) then all I have to do is lower it while swinging it 2' over. Rigging 4 or 6 at a time is a royal PITA for whoever is doing it:first one is easy, second one not too bad, third much harder, and so on, as they interfer with each other. The last time I was asked by a new customer "how many could I set at a time," I told him "all of them." Then when he said how about 6 at a time, I told him I thought he wanted to get it done quick as possible, I sure did as I had other work to get to. I used my hook and we beat the other 4 plex crew that was using a 4 truss bar by a fair bit, plus we made life easier for the rigger. The big advantage, the only one, for picking 6 at a time is for the crane, making less cycles, while the op watches the struggle to rig and to unrig it, unless they have 2 or 3 riggers wrestling them, and usually it's one.
I have cogitated on a multiple truss spreader, that used rigging that could be unhooked "from the bottom." so when I winched down it would not stop at the top chord, some sort of secure quick link, imagine 2 4' lengths of chain, both secured to the bar, with the quick link in the center of the two chains, that way when reay to unhook, the link could be lowered enough for a guy standing on the bottom chord to easily reach it. BUT, there ain't no such animal, the closet I've thought of would be a chain quick link, the ones with the screw that joins them, but again, you still have the hassle of rigging them off the stack, and wearing gloves the screwing would take too long. I've thought of using various aircraft hardware, pip pins and a clevis, but repeated abuse would make that iffy, so for me it's two at a time at most, while 99% of my customers want them one at a time, but quickly.