Disallowing self-signed certificates doesn't improve security of unencrypted connections, but it increases security of all connections using SSL/TLS. It doesn't make you more secure (you don't have strong authenticity in any of those cases), but it certainly makes me more secure.
When Gmail fetches email from my provider, which has a certificate signed by a trusted CA, it would have previously accepted any self-signed certificate from an active MITM.
When Gmail fetches email from my provider, which has a certificate signed by a trusted CA, it would have previously accepted any self-signed certificate from an active MITM.