I originally posted: Every time I have hit this message, it has been mostly irrelevant to me and disrupted what I was doing
[I'm no longer so certain - I can't be sure my router configs haven't been stolen by a MITM attack. I suppose I really ought to find out how to generate and install SSL certificates from a trusted root on them, and post them to someone at the remote sites on an encrypted pen drive.]
I manage a fair amount of networking kit, I find Google results to mailing lists with mysterious and pointless SSL connections. As someone posted in the "End of the Windows Era" thread: "I don't care what OS you have, as long as you have a reasonable browser". This isn't reasonable behaviour.
SSL does not prove anything useful - at the very most that you are connecting to the site your browser intended to connect to, assuming the site DNS hasn't been hacked.
Anyone can pay $20 and get a valid certificate and that doesn't mean you should trust them with your bank account details. Any site with a valid SSL cert might have been hacked behind the SSL termination. If you're scared of MITM attacks, aren't you just as scared of valid SSL certificates on sites with fake DNS or hacked servers?