Insurance is a good reason, ethics is another.
When light fixtures are hung properly (not over-tightened, under-tightened), fixtures falling from the sky is unlikely and rare. However, c-clamps are known to explode and bust apart without warning. Inherently, your fixtures could fall at any moment (although it's rare, it does happen). The better reason to have
safety cables is because the situation comes with a very high potential for human error. Usually a couple times a year I'll hop in a lift for a focus (after the electrics have been flown out) to find that a
fixture was placed on the pipe but the
c-clamp was not tightened down
at all.
People are human, and humans make mistakes -- often innocent mistakes. We all like to expect that our technical staff members will never put anyone in harm's way purposefully or accidentally -- we also like to think that only amateurs make those sorts of mistakes, and because we're not amateurs, we'll never make those mistakes ourselves. But I know my crew and I know myself better than that. I wouldn't classify anyone I work with as reckless, and if I ever did, I would refuse to work with them. But getting distracted during a light hang, forgetting you didn't tighten that one clamp down all of the way, and then being certain the
electric is ready to be flown up to
trim height -- that doesn't take an act of purposeful recklessness; it takes working in an environment where sometimes there are distractions that require a person to regularly
shift their attention to something else for a brief moment.
There are many better reasons to use
safety cables than to avoid legal ramifications. I trust my coworkers and our student employees, but certainly not more than I trust myself, and I know myself better than to naively think I don't occasionally forget to tighten a clamp down. That means I also know better than to think my coworkers will never make that mistake. I even know better than to think a mandate that all fixtures be inspected before electrics
fly out will make a difference (because fixtures will slip through the cracks, or at one
point in time or another someone will simply forget to do a once-over before flying everything out to
trim height).
Aside from using
safety cables selfishly as a measure to protect me from myself and my imperfections, I also care very much about the people who walk onto my
stage. By walking into my performance space where I have hung lighting, setup audio equipment, managed in any form, or built a set in, a person must have an implicit trust that I have taken the appropriate measures to ensure their
safety. If just once a
fixture falls, even if it doesn't hurt anyone and falls from 8' off of the floor instead of the usual
trim height of 25', forever after that I will have performers reluctant to walk out on
stage -- reluctant to ever trust my professional decisions ever again. They'll always be wondering in the backs of their minds what the chances are that the next time a
fixture falls that it falls on them or someone they care about. Every
theatre they'll walk into after that will also have them concerned. Their art will suffer because they'll become distracted worrying about the dangers of working in
theatre when they should be worrying about remembering their lines.
As technicians, it's both our duty to protect our performers from themselves as well as to protect them from us. It's our job to worry about their
safety so that they never have to.
The above reasons don't make good arguments for people who are worried only about paying the bills, but they make it so that I can sleep comfortably at night. In a way, they're also qualifiers for me to work for a given employer. An employer who skimps on such basic
safety necessities is not someone I would ever want to work for, so if they want me and my expertise, part of the cost of doing business with me is guaranteeing the
safety of everyone involved.