> At worst, this seems like only a minor nuisance.  The ABC metaclass is limited in its powers and seems to reasonably cover the common use cases.  
> I recommend leaving it as is.  Guido, what do you think?

Agreed. The abstractness checks are limited and not intended to prevent all ways of creating abstract instances -- just to catch typical mistakes early. (I vaguely recall another bug report about a weakness in this check that I resolved in the same way.)