There isn't any lock-in using a Helm. You can always access and export your data on the LAN using standards-based clients to get your data wherever you'd like it to be.
You say "standards-based clients", does this mean I do not have direct access to the filesystem? Or a shell account for that matter?
Using email as an example, having to transfer email with drag'n'drop in something like Thunderbird is clunky, slow but works to a degree.
Having direct access to the mailbox directory on the filesystem would make it easier to transfer out/in (assuming you already have your emails in the same format).
