History access by Up/Down is important because it makes IDLE history much more discoverable.  Most users instinctively expect Up/Down to work; moving back through the text of the session is nice but they probably won't guess that you can press Enter to reuse old commands; and it's not an efficient way to access history anyway.

The problem is increased by the fact that Alt+P/N *don't appear anywhere in the menus*, so a user will never discover them!