Unless changed, VMware Fusion can use a maximum of two cores
per virtual machine. If you are running lots of vms, then you may
benefit from more cores.
I agree with Infrared, more cores are bound to be better. Don't get virtualized cores confused with physical cores in your box. I can see in Windows running VmWare Server on an Intel Q6600 with one instance of viirtualied LINUX running, that the CPU load is being carried by at least three cores when only 2 cores are declared in the virtual machine setup. I assume that this is also the case in OSX under VmWare Fusion.