Unless your adversary has a time machine, deleting from the server protects your past e-mails from any server-side compromise.
That's not nothing. :-)
But you're right there are trade-offs. If you don't have good backups, you are indeed increasing the odds of data loss by managing the data yourself. That is also true of encryption of data at rest, you are increasing the odds of data loss to buy some protection against unauthorized access. There are always trade-offs.
I had grand visions for how Mailpile could help mitigate such issues by encrypting the mail and re-uploading back to an IMAP server. But I haven't gotten that written, so for now it's just an idea. Someday, I hope.
That's not nothing. :-)
But you're right there are trade-offs. If you don't have good backups, you are indeed increasing the odds of data loss by managing the data yourself. That is also true of encryption of data at rest, you are increasing the odds of data loss to buy some protection against unauthorized access. There are always trade-offs.
I had grand visions for how Mailpile could help mitigate such issues by encrypting the mail and re-uploading back to an IMAP server. But I haven't gotten that written, so for now it's just an idea. Someday, I hope.
Disclaimer: I wrote Mailpile. :-D