The point is not about valid signatures from untrusted keys, it's about invalid signatures from untrusted keys. Some repositories are signed with multiple keys

The point is not about valid signatures from untrusted keys, it's about invalid signatures from untrusted keys. Some repositories are signed with multiple keys. People building the repositories might only have one key T as trusted, and then mess up signing with the other key U (imagine that one is an offline key - this is how debian stable releases are signed). By verifying all signatures and rejecting any invalid one, it will be immediately obvious if the signature by U is wrong. Also, as mentioned before, the scheme allows for rapid subkey replacement in case of potentially compromised subkey.