1) It ruins your neck. The ergo people at Yahoo really dislike giving people dual monitors let alone 6.
2) You can only look at so many of these screens at once, probably too at the distance he is sitting. Why not just use virtual desktops at that point because it doesn't cause neck trauma.
By the same token, I totally understand people who want a big central screen and secondary screens either side for supporting tools/reference information. As long as the set-up doesn't have a break in the middle so I can spend most of my time looking straight ahead rather than keeping my neck permanently kinked to one side or other, I'd find such a set-up very useful.
In what way, by what mechanism, over what timescale and possibly, demonstrated significant in which study?
I've been using multiple monitors on my home computer since 2001 and at work since 2005 and I attribute my neck crunchiness to general physical atrophy due to aging and desk driving.
Usually people advocate movement, saying people weren't built to stay still for long periods, multiple monitors require head turning but not as much as walking requires leg join movements (and neck movements for that matter).
"because it doesn't cause neck trauma."
'Trauma' is a scarily strong way to describe "turning your head".
Personally I am using much less monitors, but I am very interested in ergonomics of it.
It's not that moving your neck is bad, it's that constantly moving it around all day causes unnatural stress. Much like you can give yourself RSI from bad posture when using an input device.