What leads you to believe that the second PCI will only use half of the electrical contacts? The electrical contacts for a second PCI slot are identical to the first. It is the bandwidth that is restricted: usually 16 X 0 X 0 or 8 X 8 X 0 or 8 X 4 X 4.
The reason that the bandwidth is restricted is that the Intel 1155 processor's design. It can provide only so much bandwidth.
A PLX chip provides additional bandwith to supplement the native PCI channels on the Intel 1155 platform.
The advantage of SLI is that it provides (almost) twice the graphical processor power as a single GPU. The downside is that SLI does not allow you to access additional memory.
For a single monitor the 8 X 8 X 0 PCI is fine (and so is 16 X 16 X 0) because the graphical channel is not saturated. The use of the PLX chip really is seen in multiple monitor set-ups.
A GTX 680 in SLI (2 GTX 680s) will be a very powerful graphics array that will allow you to play almost anything on ful max settings. With or without a PLX chip a single monitor will show no difference. Only when you go to muiltiple monitors will there be any perceptible difference.