[idek]

I personally don't think I'd use a tactical style mount either (I use a standard length stock). I see myself more likely hunkering down in a "safe room" waiting for LE, and focusing on a single doorway or choke point than going all G.I. Joe and dashing into combat. But there are people with different situations or mindsets that promote a more active response. And for such people, a more active posture and shorter stock make sense.

As for how it affects the way a person sights down the barrel, the butt of the gun goes more onto the pectoral muscle than in the shoulder pocket. This aligns the barrel with the eye. This may sound painful, but a person also leans forward more than usual in this stance in order to better roll with the recoil. In any event, anyone using a squared up mount would definitely want to get in some practice using the gun that way.

As for the body armor, yes, I've heard that's a reason some people use shorter stocks. Another consideration is that a shorter stock may be less likely to snag during the mount, especially if a person's wearing a night robe, a big sweater, or some other loose clothing not designed or normally worn for shooting purposes. Also, maybe it'll be a wife who has to use the gun. It is likely easier for the guy to adjust to a short stock than a typical sized woman to use an overly long one.