I'm not against the use of safety cables, but has any one actually ever seen a safety cable catch a falling instrument or be used for more than a visual safety measure? I've been doing lights for 15years now, and have never seen an instrument fall and be caught by a safety cable. I've got more use out of safety cables by using them to hang them off a ladder when hanging likes (this way i don't have to make as many trips)
Safety cables are actually quite important. Just because you haven't seen a light fall doesn't mean it isn't going to happen. C-clamps are inherently flawed devices, if you look at the way forces are applied to them and the materials they are made of it is almost like they are designed to fail. You don't have to over-tighten a C-clamp to much to weaken it, and a little bit every time you hang the camp adds up. Sure, in general we see clamps fail while they are being hung or struck, but the one that slips by could be the one that falls.
This of course is not to mention that there are a lot of other things that could cause a c-clamp to fail. A shock-load to a lighting position, say a running fly that hits the grid, could cause the c-clamps on multiple instruments to fail.
As for the comment that was made in a post about the the yoke bolts failing before the c-clamp, that also seems doubtful. On a source 4 the yoke bolts are at least grade 5 bolts, I don't know offhand what the rated strength of the bolts is, but I would have more faith in them than the C-clamp.
I have seen instruments fall, and it isn't pretty and luckily no one was hurt. As I said, just because you haven't seen it doesn't mean it won't happen. Being lax about safety is not a good practice at all.