In a large commercial barn with horses constantly coming in and out, this makes sense. With a more stable herd, the benefits of social interaction can outweigh the risks of herd living IF properly managed. Interactions through a stall bar or over a fence don’t entirely fulfill equine social needs, but can work when there are mitigating factors.
Also, it really does vary by region and discipline. I would not board any of mine at a place that didn’t do group turnout.