It sounds very exciting. But, the first example made me flinch: $ vault write secret/foo value=bar Don't ever pass passwords on the command line. On linux, other users can see your command line (ps aux), and it usually ends up in a shell's history file.
It's a tricky question, one that we've spent a lot of time thinking about at my company (Conjur -- shameless plug, one of our use cases is secrets management). Obviously doing this on a server that others can access/pwn is a bad idea, but that's a very rare use case. Most secrets are being added via the command line from an admin's single user laptop.
In the end, we've settled on what seems to be the same solution as Vault: support other sources (stdin, files, etc.), but allow the command line.
That said, I think you're right that we should look into adding a warning for the CLI tools and placing more emphasis on this fact in our documentation.