do they work without the pto shield yes. and run just fine.
on other hand "safety issue" granted no one is going to be back behind tractor when a rotatory cutter is hooked up, expecct to disconnect or connect one up. unlike say a post hole digger, were a helper may try to walk up to the post hole digger to try and wiggle / move it around.
the issue is the PTO shaft is still dangerous on a rotatory cutter. a loose shoe lace, or something else, and your ground meet. i could see more issue of dragging a air hose around to blow unit off, or a garden hose to spray unit off. and hose getting grabbed, and you getting tangled up and off you go to become ground meet.
PTO shields are meant to stop from stuff from getting wrapped around the PTO metal portions of the shaft (within reason). a PTO shield can help keep from getting grease all over ya via PTO shaft.
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with above said, for me, post hole digger, rotatory cutter, both do not have shields on there PTO shafts, and the belly mower both covers for the belts / pullies on both sides are removed. ""it is a death trap waiting to happen"" but it is what it is. my mom and myself are only ones allowed to drive the tractor. i have told other folks that i knew were good knowledge folks and use to machinery flat out no, on getting on the tractors. due to safety issues. ya i know i was rude to them but not worth risk of someone not use to machine and getting into an ugly situation.
i don't mind risking my own life. but not about to toss risking someone's elses life.