The idea is probably that is you included a criticality flag, pilots would ignore low priority notices and potentially miss something (like grass being mowed at the time they plan on landing) that could affect them, but is otherwise immaterial for most others.
I can’t say one way or the other, as I’m not a pilot, but that’s the argument I’d make to keep the system w/o priority levels.
As a pilot of single-engine airplanes, I disagree that mowing is useful. I need to look at the runway I'm landing on for obstructions no matter what. The time of year when mowing might occur, I might also have to contend with deer, who are just as hazardous and don't file NOTAMs before grazing near the runway.
I can’t say one way or the other, as I’m not a pilot, but that’s the argument I’d make to keep the system w/o priority levels.