Bandwidth isn't the only advantage of PCI Express. PCI-E will also provide hot swapping, at the hardware level. It also has the potential to lower motherboard design costs, heat issues, etc.
Not to mention that PCI-E may potentially allow for servers using multiple Infiniband 4X cards without too much trouble. That's more than 10Gbps.
Just because it ain't a problem now doesn't mean it won't be.
