Server power supplies - 6 pin to 6+2 power cables

[tf2addict]

So all of these breakout boards I've seen such as what Parallel Miner sells use 6 pin to 6+2 power cables.  PC power supplies have true 8 pin to 6+2 power cables.  Is 6 pin output a concern for high power cards like the Vegas?  I assume it's ok but there must be a reason PC PSUs provide 8 pin output instead of 6.