I don't know how much addt'l safety you get from the time and labor to ground these switches. I've never gotten a shock from a switch, in my whole long life.
That's a good question!
It would be interesting to see statistics on how many people have been shocked per year for specific things like from switches or from metal wall plates attached to the switches.
Also what the specific situation was. Perhaps you need a certain set of circumstances before the problem has happened, like a metal wall plate rather than plastic AND metal electrical box AND hot wire in electrical box touching metal electrical box.
Or perhaps in rare situations an electrical switch can malfunction / break and an internal spring can connect the metal part of the switch with hot?
Or perhaps a kid sticking an ice pick type object into the switch?
I suppose someone might have had a plastic wall plate and touched the metal screws and been shocked?
Then I once was watching a "decorate your home" type show and they showed how you can "fix up" your electrical wall plates by removing them, then covering them with a thin copper sheet. They cut the copper sheet with scissors, then folded it back around to the inside of the wall plate. And the ends of the copper sheet folded back inside wanted to spring in towards the wires in the electrical box! (I could not believe they were telling people to do this???)
Anyway I know they come up with these grounding rules because people have been shocked in the past. Maybe it is just in case some future homeowner decides to watch that TV show and cover their switch plates with copper?