I think this really only applies for signatures you can trust. A frightening number of keys used to sign software are not signed by anyone, so they're effectively the equivalent of a self signed certificate.
I've also found some projects where the key used to sign the software isn't even consistent between releases, so you don't even have the (minimal) protection offered by checking the signatures against "known good" builds.
I've also found some projects where the key used to sign the software isn't even consistent between releases, so you don't even have the (minimal) protection offered by checking the signatures against "known good" builds.