According to a VR-Zone.com report by our old colleague Lars G. Nilsson, it appears Intel is pushing for PCI-Express x2 interface simply because PCI-Express x1 just doesn't provide enough bandwidth and PCI-Express x4 is just simply too wide and complicates things, chip wise.

Intel appears to be pushing for implementation of PCI-Express x2 due to the fact that current SATA controllers and USB 3.0 controller for that matter really need more bandwidth and are currently limited to PCI-Express x1 or x4 in some cases that sounds like overkill and makes things a bit more complicated for motherboard implementation.

Of course, PCI-SIG, an association that controls PCI-Express standard has no plans for PCI-Express x2 interface support. Of course, we are talking about Intel here and things can easily change.

The other side of the story is that Intel might want to push technology in order to bring its Thunderbolt tech a bit closer to consumers, as it might be that the PCI-Express x2 is a sweet spot for Thunderbolt controllers.

More here.