Jammer Six, I have lost count of the people I've met or talked to over the years who imply the thought process of, "If I don't have a use for it, there is no legitimate use for it." Not sure if you intend to, but you are edging that way...
For people who spend a lot of time, by necessity, seated, a shoulder holster can be useful. For those with lower back issues, who find any imbalance in pressure at the waistline level to cause excruciating pain, a shoulder holster can be useful. For those who wear jackets, a shoulder holster can allow easier access than can a belt holster.
All that said, I very rarely use shoulder holsters. I prefer strongside IWB. If cold weather means I wear a zipped up coat, I keep a secondary handgun in my weakside front coat pocket. But I recognize there are legitimate needs and uses for shoulder holsters.
As far as range safety goes, training methods can offset risks. The local range I use, when I don't use my own property, has three bays. Each bay has steel lined walls and ceiling. During IDPA matches, all three bays are active, and we don't have to worry about shooting the guys in the adjacent bays. Some of the shooting we do is at targets to the side as we move; guns are to be kept greater than 90 degrees away from the back area, where the other shooters wait.
IDPA doesn't allow shoulder holsters, so we don't have to work around them during matches.
If training were to involve shoulder holsters, I imagine we'd invoke similar safety procedures as we use for "El Presidente" type drills. Determine which way the gun is likely to sweep during the draw phase (for a righty with a shoulder rig, this would be his left side; right side for a lefty; for El Presidente shooters, we ask which way the shooter intends to turn). Place the RO/SO and watchers to the side away from where the gun will sweep.
It ain't rocket science.
I'd argue that it's much more dangerous to have shooters out there who are barred from practicing using their carry method, than it is to come up with training procedures that allow them to train in a safe manner.