Welcome to the Community - The biggest features you gain by buying a license if you is the increased memory and the ability to centrally manage multiple ESXi hosts with VMware vCenter which also provides higher order funtions like VMware Hugh Availability, VMware Distrubuted Resource Scheduler and vMotion - this doc will provide additional informaiton http://www.vmware.com/files/pdf/vsphere_pricing.pdf
You can install as many free licenses as you want but you will bave to manage each host seprately -