I see snooping as a lesser concern. For one thing the three letter agency snooping somewhere on the backbone can't even see the identity of the parties involved. They will typically see NATted IPs on both ends of some service provider. Once they decode the protocol they can get a little more information but at the end of the day the difference between knowing you established an SSL connection with respect to some static site and seeing the actual content of the page that you've viewed is IMO not a big difference.

The bigger deal is the confusion it creates and the assumptions a lot of the users of self signed certs (and certs in general) make about security.

Lastly I want to point out that if your certificate is signed by some external entity that doesn't prevent you from doing out-of-band management of your public key. As long as your private key is secret you never lose any security by having your cert signed by a third party. People can argue about how secure that signature is vs. those agencies who can force the third party to reveal their secret key but while that's an important consideration, until the government sells my banking information to somebody I mostly care about this at a principle level.

WiFi attacks are not necessarily passive, ARP poisoning can redirect a victim's traffic to your machine at which point you can MITM to your hearts content. If you have trained your users to ignore the inevitable security warning (Grandma isn't going to check an SSL fingerprint) they are going to be operating under the false assumption that their communications are secure. For most people this is a worse scenario than the NSA having their data.

If you are talking about a thick client, with the appropriate checks built in, I'll agree that it's possibly more secure. Other than that, its only possibly more secure if you are the only user or you can eliminate the security warning (e.g. by distributing the certificate).

I'm skeptical that any US based company could get away with not rolling in the face of a subpoena/NSL so the protection provided by the service provider knowing they have been compromised is minimal IMO.