I'm just thinking back over the years about the dozens and dozens of fixtures that I have added and installed using these old-work boxes. I can’t remember ever installing a fixture that weighed more than a couple of pounds on these, let alone 6-pounds, but I guess I could see someone else who didn’t know any better, coming in behind me and installing some 18-pound drywall-breaking behemoth at a later date.
I don't think anyone with even limited trade knowledge or common sense would attempt to install a heavy fixture on one of these boxes, but apparently the NEC is concerned that the homeowner, handyman, maintenance person or other unqualified individual will do just that. Probably similar as to the reason why we now need to have handle ties on all multi-wire branch circuits... to protect the unqualified individuals who have no business doing what they are doing anyway.
