[andreaso]

You appear to threat trust as something binary; either you trust a host or not? In my world trust (as well as security in general) is always a spectrum, or something more multidimensional; that you trust various host to different degrees, in different way against different threats.

Let us for example say that I have a server which you are fairly certain that noone will compromise, but you do have a concern that someone might physically steal it. In such a case you might be more likely to trust the javascript it serves than you are to trust it with storing your actual private key.

(Yes, I realize that someone who gets physical access to the machine will be able to modify its code, etc. Yet, while it might be fairly easy for someone to physically break into a building it might be harder to do so without leaving any traces behind, alerting you of possible tampering.)

By the way, my trust example above is fairly similar to the use of ssh-agent forwarding; where you trust a machine enough not to abuse an active forwarding, but without having to trust it to actually store your private ssh key.

Neither do I understand why you appear to say that SSL would provide a comparably security. OpenPGP will definitely provide a stronger transport security than the possibly of there being SMTP StartTLS being done. Likewise might OpenPGP matter for the recepient, especially if that person are doing the decryption locally on a workstation/laptop, saving that person from having to trust his/her mail provider.