I think this is a duplicate of As a User, I would like to download all my encrypted passwords to local storage
The problem with the export/import functionality is that most of the formats it exports to are plaintext: the risk of users downloading a plaintext CSV file and leaving on their desktop is too high, so we disabled that function.
But the idea of having an offline mode - where the PGP-encrypted secrets are stored in HTML5 local storage, say - is an excellent one.