> They support _more than one_ set of PS1/PS2 conventions.

I saw this, not sure if that's required.  IMHO, same effect should be possible by running doctest several times with different conventions.

> Who would it help?

In principle, any project, that uses some custom shell (i.e. IPython) per default.  I believe, "easily understood tests" should start with prompts that users could easily recognise.

> Something that might change my mind:  find an author of a widely used alternative Python shell who says this change would allow them to _remove_ their own doctest-PS1/PS2 customization code.  That would be a real use case.

I admit, exactly this use case I can't provide right now.