Cases #a and #b can't be distinguished, that's why it adds basically no benefit to use encrypted only. But, you could use encrypted-only for cases say, you're a whistle-blower but don't want things to be able to be verifiably tracked back to you. The case of a service provider encrypting is probably super rare. Encryption-only gives you privacy for the one message. We just must be sure we don't trick the user to thinking it's secure, and act (e.g. transfer money or reply) on that assumption, with potentially sensitive information
Bug 1634496 Comment 17 Edit History
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Cases #a and #b can't be distinguished, that's why it adds basically no benefit to use encrypted only. But, you could use encrypted-only for cases say, you're a whistle-blower but don't want things to be able to be verifiably tracked back to you. The case of a service provider encrypting is probably super rare. Encryption-only gives you privacy for the one message. We just must be sure we don't trick the user to thinking it's secure, and act (e.g. transfer money or reply) on that "secure" assumption, with potentially sensitive information