I believe --no-prereq-checks was added to prevent setup.py from complaining about pyqtgraph not being installed, otherwise it will get stuck at a prompt asking if it should proceed anyway.  In v1.3.0 --yes seems to have the same effect.
[pyqtgraph][1] is not required to run binwalk as it is only needed when producing [plots][2] so the application will still work without it.  The only other requirements are covered by Homebrew (libmagic and matplotlib).
I suspect pyqtgraph violates the Homebrew rules since it is available from PyPI, otherwise I would submit a formula for it.

  [1]: http://www.pyqtgraph.org/
  [2]: http://binwalk.org/screenshots/