TLS is a system designed to support both authentication and encryption. Losing authentication is actually a pretty big deal, so of course browsers won't do it.

Encryption without authentication isn't really a good idea, because you can trivially MITM someone, serve them your certificate for google.com, open up a connection to google.com yourself, and it's now effectively as good as plain-text.

The security community was really upset with SuperFish and the recent Dell eRoot for the same reason -- you effectively lost accurate authentication, making it pretty easy to degrade someone's connection to plain-text.