That's definitely more convenient. I think it could be nice to have an additional test suite not written in the same language as the thing you're testing. It would force you to interact with your program the way the rest of the world would. Rather than relying on mocking, setting up test data, and reaching into the internals of your code, you have to set up your test data through the API. This wouldn't be feasible in most of work I've done in my professional career, but in an ideal world I think it could be beneficial not to rely on internals for testing, at least for some set of tests.

Yes, that’s generally a good thing to do. This tool has the potential to be much more useful for consumers of your service to document it or interact with it.