Although I agree that some design decisions forge a language and sometimes these historical developments win over sounder or more "logical" arguments.

That being said, I believe (as the OP of issue 8743) that the ABC collections should comply to the API of the Python base collections. In that sense, I would expect any custom collection implemented by extending the ABC collections to be used in place of an exiting base one, without any "hiccups". Thus, the custom collection should pass all the tests in the Lib/tests.

Of course, for the moment, this would just require extra work on my part in order to implement all the magic methods and such.

If history is to win again, then at least the documentation of the ABC  collections should clearly state the subtle, but IMHO important, differences between the two.