I explained somewhere else that a secondary prompt would be wrong for Idle since the primary prompt  asks for a *statement*, not just the first line of the statement.  The advantage is being able to edit and recall entire statements, not just one line.  This is in addition to the absurdity of entering and then removing secondary prompts from the text buffer.

I still intend to at least add an option to move the prompt(s) to a sidebar and use 4 space indents.  At that point, it would not too much matter if '>>> ' were, as an option, followed by '... ' instead of '    ', although it would not look so good with a proportional font.