thinking about e2ee chat key verification

does OMEMO not have a concept of user keys, just device keys? that seems suboptimal

since if Alice trusts Bob.Desktop, and Bob.Desktop mutually trusts Bob.Phone, there'd be no issue with treating it as if Alice trusts Bob.Phone, right?

i'm not sure how useful users assigning trust on a per-device basis as opposed to per-identity key (which every device holds a copy of. or you do cross-signing to build a chain back to the first device trusted and use that, either one works.)

do xmpp users make use of this property in practice?