What I was trying to get across in my last post is that maintaining ma ai is active rather than reactive but in answer to your question I would assume that if you are being charged then the attacker has figured out that your body mass or strength is less than his. Meeting that situation with pure irimi could prove difficult.
You don't necessarily have to get behind either - just irimi along a different line. We do drills where we avoid in one of six directions (side,front diagonal, back diagonal) followed by irimi. This works real well if uke thrusts with tanto and pulls back but you can also do it with shomen-uchi.
I don't like the use of the word defensive postion either. I think that if you have managed to avoid the attack it is required for you to resolve the situation. In other words waza the guy.