Free ESXi is not supported due to VMware restrictions in backup APIs. These restrictions were partly removed in free ESXi 5.0 (I think unintentionally), and fixed in the later ESXi updates.
Think twice before deciding to run an unpatched version of ESXi for the long period of time because of potential risks. I would keep every ESXi server fully patched.
http://www.veeam.com/vmware-esxi-fastscp.html