Description
I've found myself using matchers a lot for non-testing related purposes. It's a very clear way of describing an object, and seems more so than writing your own callable.
It also seems particularly useful as an easy way to write validators for expected JSON input, etc., if you are using it for something small and you don't want to write a full JSON schema for instance.
It might also be useful for something like the attrs library, for specifying custom validators.
The mismatch reporting is also pretty great.
It seems like a pretty useful tool for people to use, but it might get more visibility if detached from testtools. Right now the documentation is part of the testtools documentation, and people might not learn about it unless they're looking for a testing utility.
As it seems like it can stand on its own, maybe splitting it off into its own package and documentation/example usages outside of testing might encourage more usage and contributions?