I'd like to extend the Derby by two more weeks and add a fourth beta.
There's no harm in adding a fourth beta and if it improves the release
you should probably do so. We don't owe anyone a particular release
date and AFAICT no one is holding their breath. The release date is
a self-imposed deadline.
That said, further 3.4 work on the Argument Clinic ought to be aimed
at improving the quality of the work so far, rather than digging in deeper.
When it comes to conceptual flaws, the important thing is to make sure
that the 3.4 release doesn't lock you in to a decision you're going to
want to change later.
FWIW, several lines of the Zen of Python seem to be relevant
to this discussion.
Raymond