In my script, I wanted two things at the same time:
1. Setting a formatter class so that the epilog would have the line breaks preserved.
2. Telling argparse to automatically display default values for all arguments.

Currently, both things are handled by the same configuration parameter: formatter_class
This means we can either pass argparse.RawDescriptionHelpFormatter or argparse.ArgumentDefaultsHelpFormatter, but not both.

I did a hackish workaround by subclassing both formatters, and passing my (empty) subclass to ArgumentParser. It works for now, but it might break on future versions.

The ideal solution, however, would have a native support for both features, or at least explicitly allowing subclassing.
Maybe ArgumentDefaultsHelpFormatter could be transformed into a simple boolean parameter passed to ArgumentParser object (and maybe also passed to add_argument() method), instead of being a formatter class.

Yeah, adding a formatter instance seems overkill for the usual case of wanting to preserver formatting of the epilog.

> adding a formatter instance seems overkill for the usual case of
> wanting to preserve formatting of the epilog.

Related bug (look at it before (re)designing the API):
http://bugs.python.org/issue12806

Your solution is actually the current recommended solution - mix together both classes that you want to combine and pass your subclass as the parameter. This should probably be documented somewhere (and tested more).