I was really just talking about trusted parties reusing a private key (say, for an encrypted disk image shared amongst team members) not arbitrary strangers on

I was really just talking about trusted parties reusing a private key (say, for an encrypted disk image shared amongst team members) not arbitrary strangers on the 'net. That doesn't require that a server know the key; it only requires that those parties exchange a shared key in advance. It's no different than, i.e., a PSK WPA network password, or a shared encryption key on a workgroup document. You could still use a service like Dropbox as a reliable offsite backup, while doing all your crypto locally. Think of it as the "availability" service to complement your local policies that insure integrity and confidentiality.