I kind of understand the first reason, but the other 4 don't make much sense to me. The volume can't be smaller — you're still putting the same amount of computer hardware inside a frame, so why does it matter whether it's vertical or horizontal? And why "mounted risers on spacers", what's the point? When you're building a horizontal rig, the risers just sit on a support beam, by themselves — they don't need any mounting that way. I also don't understand how it is less cost and why you compare it with a "normal frame in shop" (instead of a self-built horizontal frame). Vertical rig like this requires a lot more work cause you have to fight the gravity: you have to attach the motherboard, the risers.. while with horizontal rigs the board and risers can just sit on support beams without any screws and spacers. I can agree with the first reason (although it's only relevant if you have up to 2-3 rigs, cause even with 3 rigs stacked horizontals require similar (if not smaller) footprint). All the other reasons sound strange to me.