Oddly enough we have pulled down some of the most dog poop cabinets you could imagine. Particle board. Hot glued together. A lot of them assembled dry with only a surface applied hot glue fillet between a back/side back bottom, stretchers, and so on. Stapled plastic corner blocks that look like they may be a hair thicker than a milk jug. And many of them were strong as a tank. They would have hung for years. I think makers and installers tend to over estimate hanging.
My issue with continuous backers for fastening is not for strength its for speed. We can cut in two nailers in no time (doesnt have to be super clean) and the rest of the install is mindless.
I wouldnt be worried about one of our uppers falling off the wall if it only had a single fastener (or for a narrow cab having no fastener whatsoever and relying on the adjacent cabs for its hanging). But the few minutes to cut in a nailer sure does make the install a lot simpler. You just poke a pre-drill 1.5" in from the left and right of every interior and run. No stud finder, no measuring, no allowing for face frame overlaps, you just hang.
I couldnt argue with Justins install practices for a minute. Good work. But I would much rather install a backer than every rely on a plastic drywall molly for install. I think he, and I, in that situation, would just skip the molly all together lol. When a bank of cabs is screwed together at the face frame, 4-6 screws on the average run would hold the whole thing to the wall.