wasting good bandwidth pci-e lines going to waste

question for you all, i have read that the nf4 SLI chipset supports 20 pci-e lines.. but with 4 slots or less on most boards(2 for sli videocard[8x each slot, or 16x to one and the other off], and 2 pcie 1x), why aren't manufacturers including pci-e gigabit ethernet chipsets instead of normal pci (take the asus a8n-sli, it has 2x gigabit, one is a pci one NOT pci-e)

sounds like theres 2 pci-e lines un-used that coulda been used

?? anyone seen any makers doing otherwise?