There must be many thousands of shoulder holsters in us daily and I don't recall hearing a single account of "Glock arm" or bystanders suffering any worse than other types of carry in gunfights but as is the High Road tradition those cites will soon be rolling in I'm sure.
i've personally witnessed an ND by a guy with a shoulder holster who was touching his gun while it was inside the holster, while the range was cold and people were downrange. while the bullet didn't land anywhere that would cause the incident to make the news, that was only dumb luck. it could have been very bad as it was pointed in the direction of a house with children inside and fortunately hit the ground between the shooter and the house instead.
a gun resting in a case is safe. i'll grant you that a gun resting in a holster is also safe. however, what is different is the propensity for people wearing shoulder holsters to want to show off their gun, or play with it, etc. when a guy is walking around like the one described in the OP, my first thought is one of my favorite Far Side cartoons.Being in a shoulder holster untouched by human hands is no different than the gun being in a gun case.
when somebody walks into a gun store with a shoulder holster, I leave.