the idea is that you can fit more GPUs into 1 rig. 1 card = 2 GPUs as opposed to 1 card=1GPU.
That only makes sense if space is a premium cost. Otherwise, you could build a cheap mining hull for like $300 to slap another 3x video cards into.
and that is exactly what people figured out.. and why 5830s were so darn popular. cheaper to stick in
two 5830s on a cheap machine then to buy a MB/PSU/etc that can support more cards/power.