I don't believe there is an specific performance advantage over 2 vCPUs vs 1 vCPU with 2 vCores. The biggest reason why folks would use this for CPU based licensing schemes like you mentioned.
The other reason may be if organizations wanted to take advantage of vNUMA. By default, vNUMA is enabled if you have 8 vCPUs or more but it can be enabled manually for smaller configurations.
For more info on vNUMA there are a number of great whitepapers that discuss it.
vSphere 5 Resource Management Guide: http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-resource-management-guide.pdf
Performance Best Practices for vSphere 5: http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.0.pdf
Matt