Sonatype requires the (non-SNAPSHOT version) artifacts to be GPG-signed; the public OpenPGP keys should be uploaded to a (MIT) key server.
But anyone can create any OpenPGP key with any name an e-mail and upload them to the key server. There is (as far as I know; or am I wrong?) no automatic mechanism in place to associate a particular software project/library with a particular public key. Sure, if anyone wants to check the authenticity of the artifacts, one can always request the public key from the software authors, or perhaps the key is already published somewhere (in a way that associates it with the particular software project); but since this can not be done automatically, hardly anyone ever does it.
So it seems that the whole OpenPGP-signing procedure, also by being quite technical and time-consuming, serves more to instill a false sense of security than actually providing much security to the average user.
So shouldn't there be an automatic/streamlined way to associate software projects with OpenPGP keys to make the whole thing actually secure?