The country that I live in is a highly litigious society. People will sue for ANTYHING if they think they can make some ka-ching along the way. I used to work with a guy who sued the local gas station (and won a boatload of cash!!) because he smashed his finger in their front door and broke it. His pinky finger. He was out of work for weeks and then bragged about how he sued them (and won!) for loss of wages, and pain and suffering. Surely the gas station has insurance. But that didn’t stop them from being sued.
As I said, I do not have a waiver, but it does not surprise me that some people would. I really have doubts that a release waiver will do you a tremendous amount of good in court, but some people may just like having the extra layer of security.
Anybody remember the story flying around the net a couple years ago about a farrier who was sued when the horse slipped off the hoof stand and sliced open a tendon? The horse was eventually euthanized. The story may have been on horseshoes.com, I can’t remember. Bad things happen to good people, and some people may just choose to try to protect themselves. It does not make them incompetent hacks.
I read another story on a horse board where a horse foundered after receiving vaccinations. But since the trimmer was out the SAME day as the vaccinations, the owner was blaming the trimmer for foundering her horse. She dumped he trimmer and started smearing her reputation all over the net about how she ruined her horse. Well, foundered due to vaccination isn’t terribly common, but it does happen. Especially because in this case, the horse received a multi-vaccine, and then West Nile on top of that.
ANYTHING can happen that might be out of our control. It does not make us incompetent hacks. It just means that sometimes life’s a bitch. Yaknow? 
And FYI - self-employment general liability insurance for trimmers is hard to get, and VERY expensive due to the risk involved. I contacted many companies who would not insure those active in the horse industry. I have a personal umbrella policy and it provides a modest line of coverage but there are many exceptions and exclusions.
Another line of defense is to establish an LLC or an LLP so that if a lawsuit does commence, at least only your business assets will be at risk and not your personal assets as well.
As is customary, you will attempt to discredit and trash trimmers every chance you get, and this thread is certainly no exception. 