I was going through the verification flow in my head before I wrote the OP, trying to come up with a justification, maybe DoS related, but couldn't come up with anything.

I don't think there's *any* value in validating the signature without also validating the signature is actually signing the contents, otherwise you could just steal a valid signature/event id pair from any other event and fool these partially verifying clients.

You're either trusting the relay to have verified everything, or not, partial validation doesn't seem useful at all.