You can see elsewhere where I discuss what the subscription provides. I think there's a huge difference between subscribing to companies that share customer values around privacy and security vs being at the mercy of companies looking to extract as much value from your data as possible.
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).