There is a difference between being sued, and LOSING a law suit. Granted you will lose money either way ( unless you defend yourself ) but you can also counter for defamation for example., Anyone can file a law suit at anytime regardless of how frivolous ( it may be tossed out, but it can still be attempted ). Big corporations do it all the time when they counter a lawsuit to attempt to drag it out in court so that the other person will settle or give up because they can't afford to keep paying legal fees with no end in sight.
With that said, its always good to have a waiver signed, but as others said, this does not completely mean that you are clear. You can still be found negligent if you knowingly put someone in a position that you knew there was a reasonable chance that it could cause them harm ( but figured your waiver would cover you so who cares? ). You could write a more specific waiver for a specific situation though. It all depends where and how an injury occurs as well. If a venue has faulty wiring and someone gets zapped, the venue, not you. If you tell someone to stand in a hotel jacuzzi as you dangle monolights over top of them and they end up fried, its certainly your fault, although the hotel could be sued as well for allowing it to happen. If you are chaperoning kids, then you are responsible for them ( and why would anyone ever put themselves in this situation is beyond me ) It all depends on what happens, and what you know or don't know when it happens. Even with insurance, there are max amounts they cover, there are deductables etc. So nothing is fool proof. Life is a b*tch sometimes, just try your best to avoid idiotic situations.