The Essentials Kit includes 1 year of subscription and is not offered without this. An active subscription allows you to upgrade the Kit at no cost to a newer version as it becomes available. If you decide not to renew the subscription, you can still continue to use the product, but won't be entitled to receive newer versions for free anymore.
Also, I find only "VMware vSphere 5 Essentials Kit for 3 hosts" with no indication of version 5.1.
Vsphere is licensed by the major version (i.e. 4.x, 5.x).
André