That is very obviously not the case. A self-signed certificate protects data from being intercepted and read just as well as a signed one. The only thing "valid" certs protect from that self-signed certs don't is impersonation. In the case of an intranet or local embedded device, if someone can MITM your connection, you're already screwed - and either way, your are just as screwed getting MITMd with an unchecked cert as you are with no encryption. The difference is, that without any encryption, an attacker doesn't even need to MITM you - they can just sit quietly and snoop on your packets.