Basically agreeing with what's been said already: IPython is an application that has Python (and some other things) as a dependency. So is IDLE. Python itself should ideally not be bundled with any applications -- it's a historical accident that IDLE is in the stdlib. Of course there should be a basic command prompt, but I think that the existing one based on GNU readline is fine for that purpose.
-- --Guido van Rossum (python.org/~guido)