SSH keys can restrict, control, and secure access to an ESXi host. An SSH key can allow a trusted user or script to log in to a host without specifying a password.
You can copy the SSH key to the host by using the vifs command. You can also use HTTPS PUT to copy the SSK key to the host.
Instead of generating the keys externally and uploading them, you can create the keys on the ESXi host and download them. See the VMware knowledge base article at http://kb.vmware.com/kb/1002866.
Enabling SSH and adding SSH keys to the host has inherent risks. Weigh the potential risk of exposing a user name and password against the risk of intrusion by a user who has a trusted key.