My understanding is that PRV valves are needed on well systems for the same reason you want one on a water heater. A submerged pump, with a stuck switch, is capable of running some fairly high pressures, at the very least damaging the pump. I wouldn't think a person would want to leave it off, but as I have stated many times on this site, I am not a well pro. Others might have a different view.
However, this quote from the New York site says it well: "Early in our building inspection career plumbers in our area were skeptical when we identified the absence of a pressure relief valve as a safety concern (and a building or plumbing code violation). Then in about 1984 a local plumber was killed by a bursting water pressure tank.
The water pump was a submersible unit located (remotely) inside of the water well. The plumber was leaning over the water tank repairing a fitting, unaware that the water pump had turned on, and unaware that the pressure control switch had been damaged or modified so that it failed to turn off the water pump at a safe pressure.
The water tank burst, metal fragments severely injured the plumber who, working alone, bled to death. After that terrible event all of our local plumbers took this relief valve requirement seriously."
http://www.inspect-ny.com/water/WPControlsa.htm