The thing is, it may not stay in the holster, you may need to draw it. When you draw a pistol from a shoulder holster that is in the horizontal position, you invariably point it at anyone behind you, who is in line with the muzzle, at least momentarily. In fact, you often sweep the arm under which the holster is situated. This is the main reason shoulder holsters are not allowed at many ranges (at least drawing from them is often, if not usually, disallowed). While I have worn them in the past, I stopped doing so because of having been unable to practice with them while at a range even though the ones I preferred and used were those in the vertical position. Those seem, to me, to make it less likely that the person wearing it sweeps anyone or anything while drawing as would be much more likely to happen with someone wearing one in the horizontal position. If I can find a range near me that allows drawing from a vertical shoulder holster, and if I can find such a holster that I like, I may go back to using them. That is due to arthritis in my hips and the relief I anticipate by wearing a shoulder rig that is not putting weight directly on my hips. If I do so, it will take a lot of practice to rid myself of the muscle memory of going for the draw on my strong-side hip when and if I wind up under the stress of a self-defense situation. For now though, I grin and bear it on the hip.
