It would be useful to be able to ecnrypt passwords or other secret string in a wiki site, e.g. a server administration howto also documenting the login credentials. I think ia already saw this feature in media wiki, where secret stuff then is shown users without needed rights as **. When Logged in with needed rights to see "encrypted" strings, the * are repalced with the secret.
It is important not to have the whole site encrypted, but the patz in between a .. Tag.